Sorry to other list readers about offtopic rant, but I can't stand when people attack software that I like.
PS : I do find Subject prefixes ugly too... Bu I hadn't found a more handy solution.
Don't use squirrelmail, or better yet - fix it. I suggest you replace squirrel with Zimbra, it's much better. I'm just a happy user.
Zimbra is commercial groupware suite. SquirrelMail is free webmail application. You are suggesting to replace whole user's email system with some proprietary locked product.
It works pretty well, is free as in beer, and the only "closed" parts are its Java core. A lot of other stuff, such as the JavaScript framework, and reused OSS project, are still open-source.
Main Zimbra's product is not Open Source Edition. Zimbra sells its products on annual/monthly subscription per seat basis.
Outlook, iSync, Blackberry, mobile connectors. Clustering. Backups. These are not open.
Do Zimbra admins know how their email setup works or just click on provided buttons? Can they change setup from default values without breaking it completely? Do admins have option to revert their changes, if something breaks? What happens if you deviate from standard OpenSource Edition setup and see cryptic Java errors in your logs or Zimbra web pages.
I have been using it for a year and I really beats everything else out the hands down. IMHO of course.
I've been using SquirrelMail for more than 6 years.
You are comparing apples to oranges. Zimbra is not email client. It has email client as part of whole server package. Zimbra's webmail client uses AJAX, has better integration with email system (it is designed for Zimbra) and it can't be compared with SquirrelMail. SquirrelMail is email system agnostic. It works with any email setup that has good IMAP4rev1 service. SquirrelMail has bundled client side filtering support, but use of server side filtering with SquirrelMail is recommended. Client side filtering is slow, because SquirrelMail is webmail client written in PHP without any direct access to existing email setup.
Zimbra can't replace SquirrelMail, because it also replaces IMAP, SMTP and POP services and other parts of email setup. Zimbra adds features that are not needed for standard email client.
Suggestion to tag subject of mailing list emails is a good one. Mailman can do that. I think some email clients (Outlook Express) can't filter emails by custom headers.
from other email
Having run squirrel for a 5+ years, and some of the most annoying bugs remaining unfixed (say support for national characters that actually works in real life)
Prove your claims. When these issues are related to broken MIME produced by other software? When they are related to the fact that translations are locked on charsets specific to selected language/country? When they are related to big CJK charset tables that are hard to implement in pure PHP?
SquirrelMail does not use utf-8 for historical reasons, but you can always change it to utf-8 if you want. Charset support was improved since 1.4.4. It was 3-4 years ago.
I am former SquirrelMail i18n developer and I suspect that you are spreading FUD about SquirrelMail. SquirrelMail has issues, but you and others are free to fix them. If Zimbra has issues, you will have to wait until Yahoo fixes it.