NeoMail has been abandoned by the original developers, and we're about to abandon it, too.
Take a look at Horde and SquirrelMail. Your mail is, and will continue to be, accessible in the other programs. I'm not sure if all three programs use a common address book or not.
We're going to switch to a more efficient mailbox format later this month, a format that NeoMail doesn't support. If you haven't spoken up by the 15th, we'll assume it's OK to go ahead.
Presumably, this will ease some of the mail problems that some users have been having.