Mailgun has the most flexible inbound routing of the classic senders. MCPmailer gives the agent the mailbox, the thread, and the tools.
Transactional email APIs · Mailgun claims checked July 2026
Mailgun is the developer-oriented sender with genuinely powerful inbound Routes: match on recipient or headers with expressions, then forward, store, or POST. If you want to build email plumbing, it hands you the best raw material of the classic providers.
If your product is email-heavy and inbound routing logic is your own domain logic, Mailgun is the right foundation. Its Routes can express things our fixed mailbox model simply cannot, and its validation and deliverability tooling are mature.
Control is the trade. With Mailgun you write the routing rules, the parser, the thread matcher, the store, the search index, and the agent adapter, then keep all six current. We ship one opinionated version of that stack with MCP on top, so time to an agent holding a real conversation is minutes.
A Route is a rule that fires. A mailbox is a thing that persists: it has unread state, archived state, a thread list, a filter mode, and a history. Agents reason much better about a place than about an event, because a place can be re-read when the model needs to check what it already said.
Mailgun can store inbound messages for a retention window, and you still build the searchable model your agent queries. search_inbox is Postgres full-text over the whole workspace, and get_thread returns the exchange in order, with raw MIME kept so a thread can always be rebuilt.
Our pre-send pipeline can refuse with daily_send_quota_exhausted and a reset time, or because the address is suppressed, or because a velocity tripwire fired. Those are designed to be read by a model. A 4xx from a generic API is not.
Every row is a claim we could defend with their documentation open beside it. Where they are ahead, the row says so.
If your Routes work, keep them. The case for moving is everything downstream of the route.
Move agent addresses to an agent mailbox
Point them here instead of at a Route, and delete the matching expression.
Retire the parser and thread matcher
MIME parsing and In-Reply-To linkage happen on ingest.
Keep Mailgun for bulk and validation
Address validation and high-volume product sending are not things we do.
For the agent addresses only, and only for what sits after the route: thread state, searchable history, per-agent keys, reply-first quotas, and a dashboard a non-engineer can read. Keep Routes for everything else.
At high volume, yes, and its basic plan is around $15 for 10,000 emails. We include receiving, threading, search, and a dashboard in that price, which is the comparison worth running.
For your product mail, yes, and many customers do. Agent mail should leave from the agent's own subdomain so the two reputations stay separate.
Per-identity mail rules with whitelist or blacklist mode, matching an exact address or a bare domain. Less expressive than Routes, deliberately.
Keep your transactional sender for the mail your product sends. This is for the addresses your agents converse from, which is a different job with different failure modes.
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.
More Transactional email APIs