SMTP will be fine if you just use a protocol and set up all new parallel infra. Overhead will be quite a bit higher but that's not a problem in modern world. B

SMTP will be fine if you just use a protocol and set up all new parallel infra. Overhead will be quite a bit higher but that's not a problem in modern world. But sharing the existing email infra is pretty dangerous. You never know when spam filters decide to filter your messages and turn your "reliable" message delivery into black hole.