MailCatcher runs a super simple SMTP server

A tool like this is very useful, but this one isn't being maintained anymore. MailHog isn't either.

MailPit, MailCrab and smtp4dev are modern alternatives.

https://github.com/axllent/mailpit

https://github.com/tweedegolf/mailcrab

https://github.com/rnwood/smtp4dev

What’s the easiest way to receive email programmatically without having to resort to a hosted service like SES? Is the SMTP protocol simple enough that it can be implemented in for example a serverless lambda?

The protocol is quite simple to implement. The tricky part is triggering the lambda and then holding the connection open long enough to get the message.

You can’t trigger a lambda directly via tcp. You’d have to go through a gateway. That gateway would have to hold both sides of the connection open for a pretty long time.

It would be tricky but doable.