Stunnel is an important part of my setup here.

Back when Apple's Mail on a more outdated OS X setup stopped to be able to connect to various mail servers because of Apple's own outdated SSL/TLS implementation (security.framework?) I just plugged stunnel in the middle to make things work again: Mail connects to localhost and stunnel then safely connects to the remote mail server.

While this was an important fix at that time it also provided surprisingly additional benefits. Now it was much easier to entirely block outgoing connections from Mail with Little Snitch. Instead having numerous allow directives per mailserver, just one full block. E.g. no more random config changes that break everything, because Apple decided to push some auto-config changes for well-known mail providers. No more accidental tracking pixel triggers. Also all the accounts are now just vanilla POP3/SMTP accounts rather than those with "special handling". Finally Mail became much more stable for some reason. No more long lockups when I want to open the Account settings, no more random lockups when launching the app, etc.

Now I really do not want to miss this extra layer anymore because all the bonus benefits (even if it shouldn't be needed any longer just to make SSL/TLS work again).

Over time bunch of other things (Mail unrelated) got plugged into the stunnel config too. :)