I just recently got back into Linux as a part-time driver and use this for the same purpose: https://github.com/evilsocket/opensnitch

Admittedly, I'm a noob so I would be happy to hear about alternatives.

There's also Douane: https://douaneapp.com/

No good alternatives.

I'd prefer my email client to not load external resources at all. If it attempts to do so (and I have to use a separate firewall to prevent it) I'd consider the email client defective.

Thanks for the reminder, buying it now :)