Mail should now be operational again across all domains. I ran into a ton of problems with the Exim configuration file on the new system (which uses LDAP and virtual mail accounts rather than system users), and it has taken most of today to iron out all of the problems this caused.
There are still a few quirks: SpamAssassin is somehow incapable of setting its home dir, so it cannot find the bayes database (this is not critical, but does mean that more spam than usual will be slipping through, at least until it gets fixed). Some users have received new login and/or password. If you cannot login, please catch me on IM to get your updated account information.
Apart from webmail still not being online, email should now be in perfect working order. Let me know if you discover anything that might indicate a problem.
Now, on to the next problem…