Have a look at qMail here
ダウンロード. Don't be put off by the site. I use it a lot but it's not quite FlexMail.
I've suffered the FlexMail crash but have it working now (4 days.)
Avoid downloading big messages (<500Kb.) Turn off the automatic sync at startup and use manual sync instead (IMAP Idle does work.) Download just headers or up to 5Kb of a message initially. Download the remaining portion when needed. These changes have stopped FM3 and FM4 from crashing for me, so far.
Note that if FM does crash, you'll need to reset your iPAQ and remove FM and your mail before re-installing.