A newsgroup reader
Ranked in these QuestionsQuestion Ranking
Pro It supports IMAP and POP3
… but you can use DavMail to connect to Exchange server via EWS.
Pro Everything is customizable
Yep, really. From Outlook-mode, where Outlook-typical TOFU mails are “repaired”, up to a high-sophisticated scoring and SPAM filtering, footer lines with quotes, rules depending on the recipient, MIME formatting, boxquotes, etc.
Con Difficult documentation
Originally a newsgroup reader, so its documentation uses Usenet terminology for everything -- which is confusing.