Well the internet down scenario has only happened once, and I returned home to no internet, booted up my laptop, and could not connect to any of my services since I couldn’t reach my control server. I haven’t forced the issue to occur by disconnecting my internet and testing connectivity. I just did the lazy thing and connected to the services I wanted via their IPv4 address
I’ve been playing with Stalwart-Email as a combined SMTP/IMAP server. Its open source and written in rust, still pretty early in development and I haven’t played with it enough to give any real opinion on the pluses or minuses compared to other software, but its worth taking a look at.