# AgentMail vs MailSlurp Source: https://wraps.dev/versus/agentmail-vs-mailslurp Both hand your code an email address over an API. One was designed for AI agents that need to hold a conversation; the other was designed for test suites that need a throwaway inbox, and it shows in the send limits. The superficial description of these two products is identical: call an API, get an email address, receive mail at it, send mail from it. The difference is what each was built to survive. MailSlurp came out of test automation, where inboxes are created by the thousand, live for the length of a CI run, and almost never send. AgentMail came out of the agent boom, where one inbox represents a persistent identity that will hold a real conversation with a real person. That origin decides almost everything else. ## At a glance |   | AgentMail | MailSlurp | | --- | --- | --- | | What it is | Hosted inbox API built specifically for AI agents | Programmatic inboxes over an API, with SMTP and IMAP | | Published pricing | Free at 3 inboxes, 3,000 emails/mo and 100/day. Developer $20/mo for 10 inboxes and 10,000 emails/mo. Startup $200/mo for 150 inboxes and 15,000 emails/day, with monthly volume quoted. Enterprise is custom. Overage rates are not published. | Free sandbox: 1 permanent inbox, 500 inbound and 25 outbound emails. Pro $49.99/mo for unlimited permanent inboxes, 5,000 inbound and 500 outbound. Team $129.99/mo. Overage $0.99 per 1,000 emails. | | Best for | Agent builders who want an inbox from a single API call and no email infrastructure to run. | Agents needing many inboxes on demand, reachable over REST, SMTP, and IMAP alike. | | Watch out for | Plans meter inboxes before volume — Developer caps at 10 — so a fleet outgrows the tier long before the send limit. Mail sits in their storage, and no compliance certification is published. | Built for test automation first, so outbound allowances are small — 500 a month on Pro — and being a production sender of record is not what the product optimises for. | Prices are each vendor's published list prices, verified August 2026. Check the vendor's own page before you sign anything. ## Where they actually differ ### What each product was built to survive AgentMail AgentMail assumes the inbox is an identity. An agent has an address, people reply to it, the thread has history, and the expectation is that a human on the other end cannot tell the difference between this and a colleague's mailbox. The design pressure is on threading, on replying correctly and on being a legitimate sender, because the counterparty is a person rather than an assertion in a test. MailSlurp MailSlurp assumes the inbox is a fixture. Create it, wait for the message your application under test just triggered, assert on its contents, throw it away. The design pressure is on creation speed, on polling and waiting primitives, and on never colliding between parallel test runs. It is very good at that, and being good at that does not imply being good at the other thing. ### How much you are allowed to send AgentMail Sending is the product, so the allowances scale with the plan in the way you would expect from a mail vendor, with volume rather than mailbox count being the thing you eventually renegotiate. An agent that answers every message it receives roughly doubles its own traffic, and the pricing model anticipates that. MailSlurp Outbound is deliberately small. The Pro tier's monthly outbound allowance is in the hundreds, because a test suite verifying a password-reset flow receives constantly and sends almost never. Overage exists, but a product whose plan copy counts outbound in hundreds is telling you clearly what it expects, and an agent fleet that replies to customers will exhaust that in a week. ### Protocols, and what can talk to it AgentMail A REST API with webhooks, which is the right surface for an agent runtime and is a narrow one. If something in your stack speaks only SMTP or only IMAP — a legacy connector, an off-the-shelf tool, a client somebody wants to point at the mailbox to look at it by hand — you are writing an adapter or you are not doing it. MailSlurp REST, SMTP and IMAP together, which is unusually broad and is the strongest reason to pick MailSlurp for something it was not designed for. You can point an existing mail client at an inbox, integrate a tool that only knows how to relay over SMTP, and use the REST API from your tests, all against the same mailbox. ### What the plan actually meters AgentMail Inboxes before volume. The lower tiers cap how many mailboxes exist rather than how much they send, so a fleet that spawns an inbox per task outgrows the tier long before it approaches the send limit. That is worth modelling before you adopt it, because the natural agent architecture and the natural plan ladder point in opposite directions. MailSlurp Permanent inboxes are unlimited above the free tier, with the metering on message counts instead. For high inbox churn that is the friendlier model by a wide margin, and it is exactly the shape you want if every task in a queue gets its own address. The constraint simply moves to the outbound number, which is the one you will hit. ### Being a legitimate sender AgentMail AgentMail is positioned as a sender of record, which means domain authentication, reputation and the ordinary obligations of sending mail that people will reply to are part of what the product is for. That matters more than it sounds: an agent whose mail lands in spam is not a degraded agent, it is a non-functional one, and nothing about the agent framework fixes it. MailSlurp Test-automation tooling is under no pressure to keep a warm sending reputation, because in the intended use nobody receives the mail at a real mailbox. If you send production traffic from an address on a shared testing platform, you are relying on infrastructure that was never optimised for the thing you are doing, and you will find out at the worst moment. ### Threading and conversation state AgentMail Threading is a first-class concern: replies are associated with the conversation, and the model your code sees is closer to a thread than to a list of independent messages. For an agent, that is the difference between a coherent exchange and a series of unrelated messages that happen to share a subject line. MailSlurp The model is message-centric, which is correct for assertions and awkward for conversations. You can reconstruct a thread from the standard headers yourself — the identifiers are all there — and reconstructing threading correctly, including the clients that get the references header wrong, is a surprisingly deep rabbit hole to go down for something you expected to be free. ### Where the mail lives and who can answer for it AgentMail In their storage, under their policy, and there is no published compliance certification to point at. That is worth knowing before an agent starts handling anything a customer would consider confidential, because the question will eventually arrive from somebody's security review and it needs an answer that is not a shrug. MailSlurp Also in their storage, with the extra wrinkle that a platform designed for disposable test data is not making the same durability promises a mail provider does. Treat anything you need to keep as something to copy out promptly, and assume the platform's retention is tuned for a CI artefact rather than a business record. ### What each costs you when the product changes AgentMail A young company in a young category, which means the roadmap is moving quickly in your favour and the terms can move too. The mitigation is the ordinary one: keep the inbox abstraction thin in your own code so that swapping the provider is a module rather than a rewrite. MailSlurp An established product whose centre of gravity is somewhere else. The risk is not that it disappears, it is that the features you depend on for a use case it was not built for stay where they are while the test-automation features improve. You are a guest in someone else's roadmap. ## Pros and cons, side by side ### AgentMail Strengths - Threading is part of the model your code receives, so a reply keeps its context without anybody reassembling a conversation out of raw headers. - Being a legitimate sender is the supplier's concern as well as yours, which for correspondence a person will read is most of whether the thing works at all. - Outbound scales with the plan the way a mail vendor's does, so an agent that answers everything it receives is an anticipated shape rather than an overage. - An address is a durable identity, which is the right abstraction when the counterparty expects to write back next week and reach the same correspondent. Trade-offs - The surface is an HTTP interface and webhooks, so anything in your estate that only knows how to relay over SMTP needs an adapter somebody writes. - Lower tiers cap how many mailboxes may exist, which points in precisely the opposite direction from an architecture that spawns an address per task. - There is no published certification to hand a security reviewer, so that meeting becomes a conversation about intentions rather than a document exchange. - Both the company and the category are young, so the terms you signed and the roadmap you liked can each move inside a single year. ### MailSlurp Strengths - Creating and destroying mailboxes is effectively unmetered above the free tier, which is the shape you want when every job in a queue deserves its own address. - Three protocols reach the same mailbox, which is unusually broad and is what lets the product cover work it was never designed to do. - The blocking wait-for-a-matching-message primitive is better developed here than anywhere else, and it is what makes an email test tolerable instead of flaky. - A permanent mailbox and a disposable one come from one API, which matters when a staging environment and a build runner have different lifetimes. Trade-offs - The monthly outbound allowance on the ordinary paid tier is counted in hundreds, which a fleet that answers customers will exhaust inside a week. - The model is centred on the message, so reassembling a conversation is your problem and so are the clients that mishandle the reference header. - Nothing about a platform built for disposable fixtures is making the durability promises a mail provider makes about a record you may need in a year. - You are a guest in somebody else's roadmap, so the capabilities you lean on for an unintended purpose stay exactly where they are while the intended ones improve. ## Pick in thirty seconds ### Choose AgentMail if - Your agents hold real conversations with real people and the outbound volume is comparable to the inbound volume. - Threading matters, because a reply that loses its context reads to the recipient as a broken system. - You want the vendor to be responsible for being a legitimate sender rather than treating deliverability as your problem. - The inbox is a durable identity for an agent rather than a fixture created and destroyed inside one job. ### Choose MailSlurp if - The inboxes are for testing — signup flows, one-time codes, password resets — and almost nothing needs to be sent. - Something in your stack speaks SMTP or IMAP and is not going to be rewritten to speak REST. - You create and destroy inboxes at high churn and want that to be unmetered rather than the thing your plan caps. - You need a permanent inbox and a disposable one from the same API, because staging and CI have different lifetimes. ## What actually moves when you switch In roughly the order you would do it. The expensive parts of an email migration are never the send call. 1. Count the outbound traffic your agents will actually produce, including the half nobody forecasts. An agent that answers what it receives roughly doubles its own volume, and one that follows up unprompted more than doubles it. Take your expected inbound, double it, add the scheduled nudges, and hold that number against the plan's monthly outbound figure rather than against its mailbox count. A product whose plan copy counts outbound in hundreds is telling you plainly what it expects of you, and the arithmetic takes five minutes. 2. Work out whether the plan meters mailboxes or messages, then hold that against the architecture you were going to build. One inbox per agent and one inbox per task are very different shapes, and the natural agent design — an address for every conversation, created on demand and kept for as long as the thread lives — collides head-on with a tier that caps how many may exist. Decide the architecture first and pick the meter that matches it, because bending the design to fit a price band is a decision you will be living with in eighteen months. 3. Put the agents on a domain you control before the first message leaves, because that is what makes everything afterwards recoverable. Addresses on a supplier's domain are borrowed: the counterparty's reply routes somewhere you do not own, the signature you have been building is somebody else's asset, and a change of supplier invalidates every address a customer has saved. Authenticate your own domain, delegate a subdomain to the agents, and treat the supplier as a thing that sends on your behalf rather than as the place your identity lives. 4. Decide how correct threading needs to be, then find out who implements it. Placing a reply inside the right conversation means echoing the identifier of the message being answered, carrying the chain of prior identifiers, and coping with the clients that truncate or reorder that chain — and a surprising number do. One side hands you a conversation object and absorbs the work. The other hands you the headers and your good luck. It is the difference between a coherent exchange and unrelated messages sharing a subject. 5. Find out what retention you are getting and start copying anything durable out as it arrives. A platform built around fixtures tunes storage for a build artefact, and a platform built around agents is young enough that its policy may not have been tested by anybody asking for a message from last spring. Either way, correspondence with a customer is a business record, and the wrong place for it is a tool whose retention you do not control. Write the copy into the webhook handler on day one; retrofitting the months you did not keep is impossible. 6. Inventory what in your stack is capable of speaking only one protocol, because that single fact can decide this. A legacy connector that relays over SMTP, an off-the-shelf tool that wants IMAP credentials, or a support person who simply wants to point a mail client at the mailbox and look at it are all trivially satisfied by one of these products and an adapter project on the other. Make the list before the evaluation rather than during the integration, since the item that catches teams out is never the main application. 7. Plan the sending posture as though a mailbox provider is watching, because one is. What gets automated correspondence blocked is not that a machine composed it — it is unauthenticated mail from a brand-new identity arriving at a rate no human exchange would produce, aimed at people who did not ask. Authenticate the domain, keep outbound roughly proportionate to what comes back, ramp a new address over weeks rather than hours, and make sure a person who replies asking for a human actually reaches one. Those four habits matter more than any feature on either side. 8. Prepare the answers a security review will want, since this is the question that stalls agent projects with real customers. Where is the mail stored, in which jurisdiction, who at the supplier is able to read it, how long is it kept, and what certification backs any of that. A young supplier may have honest answers that are thinner than an enterprise buyer accepts, and a testing platform may never have been asked. Get the answers in writing during the trial, because finding out during a procurement review means the project waits on a document that was never going to be produced quickly. ## Questions to ask before you sign Put these to a sales rep, or find the answer in the documentation before anyone quotes you a number. 1. If every agent answers what it receives, what is our real monthly outbound, and where does that sit against the plan? 2. Does the tier cap how many mailboxes exist or how many messages pass through them, and which matches the architecture we drew? 3. Are the agents sending from a domain we own, so that a change of supplier does not invalidate every address a customer has saved? 4. Who implements threading if the product does not, and has that person looked at how a real client mangles the reference chain? 5. How long are messages kept, and are we copying anything we might need next year into storage we control? 6. What in our estate can only speak SMTP or IMAP, and would we be writing an adapter for it? 7. Where is this mail stored, who at the supplier can read it, and can we have that in writing before the security review? ## Common questions ### Can I use a test-automation inbox service in production? You can, and the failure is gradual rather than immediate, which is what makes it a trap. Sending works until volume grows past an allowance sized for test suites, deliverability holds until a mailbox provider looks at the reputation of infrastructure it associates with automated traffic, and retention is fine until you need a message from four months ago. None of those break on the day you launch. All of them break later, at once, under load. ### Why do agents need their own address instead of a shared one? Because replies have to route back to the right agent, and because a shared address makes every reply an ambiguity your code has to resolve from the body of the message. A per-agent address turns routing into an addressing problem, which is solved, rather than a parsing problem, which is not. The cost is that you now have many identities to authenticate, warm and monitor, which is precisely what these products exist to absorb. ### How do I stop an agent from being treated as spam? Authenticate the domain properly, keep the volume proportionate to the replies you receive, and make sure a human can reply and reach a human. The thing that gets agent mail blocked is almost never the fact that a machine wrote it — it is unauthenticated sending from a fresh identity at a rate no human correspondence would produce, to recipients who never asked. Mailbox providers are pattern matching on behaviour, not on authorship. ### What happens to a conversation if I change providers? The addresses change unless you were sending from your own domain, which is the argument for doing that from the beginning. Message history has to be exported while you still have access, and threading will break across the boundary because the identifiers your new provider generates have no relationship to the old ones. Plan the cutover at a quiet moment and accept that in-flight conversations are the casualty. ### Can I just run my own inbox with IMAP against a normal mailbox? For one agent, yes, and it is a perfectly reasonable place to start — a real mailbox at a provider you already pay, polled over IMAP. It stops working at about the point you want the tenth one, because provisioning becomes a manual step, the polling model gets expensive in latency, and consumer mailbox providers actively rate-limit automated access. The API products exist because that curve is steeper than it looks from the first agent. ### How much outbound will a fleet of agents actually generate? Considerably more than the inbound figure people start from, and the multiplier is worth calculating before you choose a plan. An agent that replies to what it receives at least doubles the traffic of the conversation it is in. An agent that follows up when nobody answers adds a second message per thread that went quiet, which in practice is most of them. An agent that confirms, summarises or acknowledges adds a third. A realistic rule is to take your expected inbound volume, multiply it by two and a half, and treat that as the floor rather than the estimate. Then hold it against the outbound allowance on the tier you were considering, not against the mailbox count, because the mailbox count is the number that looks generous and the outbound number is the one that stops you. A product whose paid tier counts outbound in the hundreds per month is not being stingy — it is accurately describing a workload where inboxes receive constantly and send almost never, which is the opposite of what a fleet of conversational agents does. ### Why is threading harder than reading a header? Because the header is a chain rather than a pointer, and the chain is maintained by clients of wildly varying quality. Correctly placing a reply means echoing the identifier of the message you are answering and carrying forward the accumulated list of everything before it, so that the recipient's client can rebuild the conversation. Some clients truncate that list when it grows. Some rewrite the identifier entirely. Some forward a message in a way that breaks the link deliberately. Mailing lists rewrite both. So a naive implementation works perfectly in your own tests, works against the two clients you checked, and then produces an agent that appears to start a fresh conversation every time it answers somebody using an unusual mail application — which reads to that person as a system that is not paying attention. A product that models the conversation for you has absorbed this; one that hands you the raw headers has not. It is a fine thing to own if you know you are owning it, and a genuinely unpleasant surprise to discover in week three. ### How long do these keep my messages? Long enough for the use case each was designed for, which may be nothing like long enough for yours. A platform built for test automation is storing what is effectively a build artefact: interesting for minutes, occasionally for days, and of no value once the assertion has passed. A platform built for agent correspondence has a stronger reason to keep things and is usually young enough that its retention policy has not yet been stress-tested by somebody asking for a thread from eleven months ago. The right posture is the same either way and it costs almost nothing at the start: treat the supplier as transport rather than as an archive, and write every inbound and outbound message into storage you control as it arrives, from the webhook handler you were building anyway. Correspondence with a customer is a business record, sometimes a legal one, and the moment you need a message from last spring is invariably the moment you discover what the retention window really was. ### Should agents send from my domain or the vendor's? Yours, and this is the one decision on this page that is much cheaper to make at the beginning than at any later point. An address on a supplier's domain is borrowed in every sense: the reputation you accumulate belongs to them, the replies route through infrastructure you do not control, and the address a customer has saved in their contacts stops working on the day you change supplier. An address on a subdomain you own inverts all three. The reputation is yours to build and yours to keep, the authentication records are in your own zone, and a change of supplier is a configuration change rather than a mass notification to everybody you have ever corresponded with. The extra work is small — delegate a subdomain, publish the authentication records the supplier asks for, and ramp the volume over a few weeks rather than starting at full rate. The alternative is a fleet of agents whose identity is a line item on somebody else's invoice, which is a strange place to have put the thing your customers reply to.