I second Mailcow. I use it on a residential IP and have a smarthost for relaying in/out since my isp blocks port 25.

For relay, I use mxroute.com, I have a lifetime account and Mailcow has a fetchmail option.

If you are concerned about TruNAS, go look at Xigmanas. This is the original FreeNAS project before iX acquired the name.

I have been using Tabby for a couple of years and I can recommend it

https://tabby.sh/