Shared mail folders with the ability to set up proper read/write ACL's.
Delegation
Undelete messages
Delayed sending (Send at yyyy-mm-dd hh:mm:ss)
Import mail via IMAP/POP3
Export mail via IMAP/POP3
An alternative LDAP interface for reading/writing contacts
Thread muting so you can ignore entire threads including messages that arrive after you've muted it.
The ability to view the SMTP logs for a message in Sent Items to see if/when the remote server accepted the message.
RSS folders.
Message filtering in the Sieve format
PGP support. Public keys can be stored server side and used for signature verification and to encrypt outgoing mail. To perform operations that require access to the private key such as decrypting incoming mail, and signing outgoing mail you'd have to install a browser plugin which interacts with the website. Not ideal, but the only "safe" method.
100% SSL encrypted browser sessions.
