If you have your own mail server and view your mail locally on a client like thunderbird, you can’t really view your mail on any machine in which you don’t want to be installing an email client on or one that isn’t yours. Are there any self-hosted webmail options in which you can log into your email on any machine without using a 3rd party service?
I used Squirrelmail briefly. It had a minor security bug which was 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.