TabMail      

Automatic attachments compression Warn when open (or save) dangerous attachments Message filtering (input and output) using regexp Spam filtering using SpamBayes additions Simple view of compound messages Message-thread can span many mailboxes HTML messages view as plain text SMTP and POP3 authentication Support for SSL/TLS connections Scanning attachments for viruses Tracing for diagnosing TCP, POP3, SMTP or filter errors