FlexMail is great until it falls over; you'll need to reinstall it and download everything again if it fails.
FM 4 has only crashed catastrophically once for me but it regularly locks up with the spinning 'wait' icon showing. I have to kill it before I can continue working.
I consider myself an e-mail power user and sometimes need to carry 1000s of messages. FM crashes too often to be my only solution. Note that the built-in client isn't that much more stable in my experience. I use IMAP, so that my mail is relatively safe, and I have qMail loaded as a backup. Despite the appearance of the site, qMail is excellent, and did I mention that it's free? It's here
ダウンロード.