I have used squirrel mail before, works ok, just so ugly ... But since you said you are will to skin ten this is not an issue.
Want to go personal. Never call some software ugly. Roundcube can get bad review too.
Sorry dzekas, it really is ugly. I know you have a soft spot for it, and it does work. But I don't understand that a webmail client like squirrel mail with the developer network that it has, can even think leaving squirrel mail looking like it was developed back in the 80's is a good idea. You will also notice that I said with skinning it should not be an issue.
SquirrelMail does not have developer network and developers who code for SquirrelMail are not graphic designers. It was started before AJAX frenzy and rewriting to AJAX is not an option. Rewrite requires knowledge of completely different scripting language. People, who skin SquirrelMail, don't contribute back to SquirrelMail. They create own products, sell them and give back only when they are forced to. Their only "contribution" might be the push to add code modifications that allow them to continue selling their products without any further contributions. Those skins provide only limited customizations, break existing code and SquirrelMail frames might be difficult to "skin", if user wants integration with other website.
"ugly" is personal opinion. cow can look ugly to you, but I don't think that you can tell that to bull during mating season.
"plain", "outdated look", "simple", "lack of eye candy", "focused on features instead of looks". I would not argue about these, but I do flame out on 'ugly'. I have selected SquirrelMail after trying couple of other packages in 2001 and same software remained my primary email client for 10 years. It is not ugly.