1 Thunderbird 3
2 Good plugins
Tested and adopted:
Great smart view mixing accounts.
Multithreaded email and IMAP caching and management.
Sum-up of discussions.
Most Thunderbird2 plugins are compatible as long as one disable version checking.
can be customized
: partly hide quoted discussions when they exceed some length.
: manage emails storage/copying/moving through keyboard shortcuts
: sort discussions by latest messages
: calendar and agenda
: select and open long multi-line URL
Silvermel, gorgeous theme
: add links in newsgroup discussion to Google Groups.