Opera Mail powerfully organizes, indexes, and sorts your messages.
It has spam filtering (both automated and Bayesian), supports POP3 and IMAP, newsgroups as well as Atom and RSS feeds.
Opera Mail uses one database that keeps an index of all mail, and sorts the messages automatically into several "views" or accesspoints. Messages are automatically sorted by type, such as mailing lists, and mail with attachments . This approach to indexing allows for quicker access to messages. For instance, a message sent to a mailing list with a word document attached will appear in both the "Documents" attachment view and in the "Mailing lists" view. Opera Mail can also use Bayesian filtering to automatically sort messages into other views. All messages in the database are accessed by opening the "Received" view.