Here's exactly what I did. I composed an email message while the phone was in "Offline" mode. I was in a hospital visiting someone and it had to be off, or so they said. I figured that when I got into the lobby and turned it back on my email would send. Bzzzzzzzt! Wrong. My E71 locked up. Pressing the keys made the beeping sound as if it was responding but nothing moved. The power button was non-responsive as well. I pulled the battery for 30 seconds and rebooted. Seemed fine for about 10 seconds, and during that time I was able to open the main menu and navigate to "Settngs", but when I tried to select it I found it was frozen again. All the while I had the "mail in outbox" icon in the upper right corner of the screen. I repeated the process one more time, only to have it freeze up yet again. The third time I restarted I quickly opened Mail, immediately went to my outbox, and deleted the que-ed email. Everything was back to normal without even another restart.
So the moral of the story is: Don't compose email while the phone is "Offline".
I do seem to recall reading about this on an earlier phone, and it may not have been a Symbian phone. Could have been a Palm Treo with Windows Mobile. I can't recall. I just know now that email and "offline" status is bad?.