As someone with multiple mail accounts (having my own domain and mail server) I find the UI
for managing those multiple accounts with a lot of folders in each account (everything is
IMAP) to be tremendously inadequate. Thunderbird is in desperate need of a better UI to handle
how you access different boxes and folders. I also feel personal folders aren't handled in the
best way. Backup is another issue that should be much easier than it is and for god's sake
dump mbox for mdir, it's not hard to have over 2GB of mail these days.
And the spam filtering is about as smart as dirt is.