I used Squirrelmail briefly. It had a minor security bug whic was fairly easy to fix, but when I reported it to the devs, I couldn’t convince them that it was actually a bug. I decided that they weren’t paranoid enough to be working on that type of software, so I stopped using it.

Currently I’m not self-hosting email but am using mxroute.com which has a FOSS mail client that seems ok. I can’t check right now what it is, but maybe later.

Fastmail’s webmail is pretty good and they said something a while back about releasing it as FOSS but idk if that has happened.

Right now I mostly use Thunderbird rather than webmail. It sucks in many ways but I’ve had too much going on to pursue alternatives.

I think Google got it right early on when they realized that email clients should be backed by a serious search engine. The search features of a typical IMAP server aren’t enough and the one in Thunderbird is crap. So I think this is an area where FOSS clients could use some work, if it hasn’t already been done.