K9 is an experimental fork of the core Android POP/IMAP client. Primarily, K9 is focused on making it easy to chew through large volumes of email.

Release notes:

Added help messages in Message and Folder List views, shown when the ‘h’ key is pressed “Secret” debug screen now shows version number. (As opposed to version name, shown in the about dialog, and SVN revision, which is what was formerly and often inaccurately shown there.) Faster (“instant”) message deletion – danapple0 All message operations can be done on messages in the Outbox – danapple0 Deleted messages in Trash now have full body – danapple0 Deleting a message in Trash deletes it permanently – danapple0

http://code.google.com/p/k9mail/