Dave.

There was a good guide by Linuxbabe on building an email server from scratch with all the bits and pieces and antispam/email verification stuff you need to send mail to the big players, I used it a few years ago to do my server.

Here’s the collection of various guides for various ways to do it:

https://www.linuxbabe.com/category/mail-server

Yeah you also need a vps. Home addresses are pretty much all marked as spam generators these days, and most ISPs proactively block all the common inbound ports for mail servers.

Have you seen some of the stuff at Adafruit?

A lot more DIY but a wide variety of stuff. I had a MagTag unit from them that ran for a few months at a time off a 1000mAh battery with hourly display updates over wifi.