Fastmail has the best mailbox API of any human email host. MCPmailer is a mailbox built for an agent rather than a person.
Human mailboxes · Fastmail claims checked July 2026
Fastmail is the enthusiast choice: fast, private, no advertising, and the primary force behind JMAP, which is a genuinely modern mailbox protocol rather than IMAP with a hat on. It starts around sixty dollars a year for one person and has no free tier.
If a person is going to live in this mailbox, Fastmail is excellent and JMAP plus an app password is a pleasant way to automate around it. For a personal assistant working inside your own mail, it is a good pairing.
JMAP is a better mailbox protocol; it is still a mailbox protocol. It has no concept of an agent identity, a cold-send quota, a blocking wait, or a dashboard for someone supervising the agent, and every agent is another subscription.
You get efficient sync, push, and structured objects, which is a real improvement on IMAP. What you do not get is any notion that replying to an inbound thread is safe while emailing a stranger is not. That distinction is the whole basis of our quota model.
Fastmail is subscription-only, so trying an agent against it starts with a card and a per-user plan. Ours is 3,000 emails a month across three inboxes with no card, which is the difference between evaluating an idea and committing to one.
Aliases are plentiful, but an alias is not an identity: mail to an alias lands in the same mailbox, under the same credential, with the same history. Separate agents that can be revoked separately means separate accounts, which means separate subscriptions.
Every row is a claim we could defend with their documentation open beside it. Where they are ahead, the row says so.
The split is the same as with any human host.
You keep Fastmail
Your own mail, your own aliases, your own client.
Agents get their own addresses
On a subdomain, with a key each, and nothing shared with your personal mailbox.
The agent can still write to you
It emails you like anyone else, and the thread stays readable in the dashboard.
Yes, and it is one of the nicer mailbox APIs to work against. You still build threading policy, cold-send controls, and supervision, and you pay per account.
Not if you want them isolated. Aliases deliver into one mailbox under one credential, so revoking one agent or auditing it separately is not possible.
Fastmail is excellent on privacy and we would not claim to beat it for personal mail. For agent mail our position is narrower and specific: we never read message content and never train on it.
No. The interfaces are MCP and a REST API, because both are what an agent can actually use without a MIME library.
Your people keep their mailboxes. Your agents get their own, on a subdomain, with keys you can revoke one at a time and quotas written for software.
The free tier is 3,000 emails a month across three agent inboxes, no card, with receiving, threading, and search included. Enough to watch a real conversation happen before you decide.
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