It is however not always feasible to have Thunderbird around. Time to add an IMAP web client! Candidates are IMP, Squirrelmail and Roundcube. All of these projects continuously release security updates. So unless you go for the next (beta) Ubuntu release, the packaged version is almost always a few versions behind.
In the past I have already used IMP and Squirrelmail. IMP has a nice UI but was difficult to install. Squirrelmail looks really old, but has a huge amount of nice plugins.
So it became Roundcube this time. Roundcube is not out of beta for that long. But it does look really slick, almost as if it is a desktop app. I did go with the Ubuntu package after all. Lacking more documentation I had to figure out for myself that I needed to use /etc/roundcube/apache.conf as the basis for a virtual host file in the /etc/apache2/sites-available directory. After some smaller configuration tweaks in /etc/roundcube/main.inc.php I had the application as I wanted it.
One more thing: Roundcube (like IMP and Squirrelmail) continuously open and close new IMAP connections. This makes the site amazingly slow. The fix: package imapproxy. This package starts an IMAP server that caches IMAP connections to another IMAP server. Some small config changes, and Roundcube was a lot quicker.