i'm reasonably sure I have this applied to my phone, as I've only purchased it a couple of days ago and when I check windows update, there seems to be nothing available, but other Windows Mobile 6.1 users may want to check this out:
Download details: WM 6.1 Hot Fix for Sending POP and IMAP E-mail
Download details: WM 6.1 Hot Fix for Sending POP and IMAP E-mail