Thunderbird gives you IMAP/POP support, a built-in RSS reader, support for HTML mail, powerful quick search, saved search folders, advanced message filtering, message grouping, labels, return receipts, smart address book LDAP address completion, import tools, and the ability to manage multiple e-mail and newsgroup accounts.
Thunderbird 3 Beta is a preview release for the next major version of Thunderbird. It is being made available for testing purposes only. Features in Thunderbird 3 include:

* A new Activity Manager
* An Archive feature similar to the one in GMail
* A new global database