I would recommend email for this. It’s a text-based protocol and the original RFCs 821/822 are pretty straight-forward. There are some additional rabbit holes related to content encoding, but if one can implement a simple MTA, a huge amount of the magic of the internet becomes accessible.

I would not recommend trying to build a “production grade” MTA, as there is a lot of minutia to get right, and it’s easy to screw up.