Email built for the way software works now.

Every product runs on email — signups, receipts, resets, outreach, the reply that comes back. Most teams assemble that from a sending API, a mailbox someone logs into, and a suppression list they wrote themselves, then hope nothing sends to the wrong person.

Mailstack is that whole plane as one system, built so the expensive mistakes are impossible by construction rather than discouraged by a note. A campaign can't leave from the domain your receipts use. An automated agent can reach you and no one else. An opt-out is forever.

We built it to run a real business's mail first, and hardened it the expensive way. Now we're opening it to teams who need the same thing: a mailbox their people — and their agents — can operate without it ever misfiring.