Clive I have just had the same problem, first wiff of trouble was when I recieved a confirmation of payment taken from my ISP [Plusnet]The whole system froze, only way out cold boot.
Anyway Plusnet always send me confirmation emails twice, one using my email address with them and one with my Freeserve email address. Sure enough as the second email attempted to come in the system locked up again, just got the busy hour glass. I noticed the OE envelope in the tool bar was white and opened with a blue tip on the top of the open flap.
Anyway as I had experianced the prob when the first Plusnet email arrived I guessed this incoming was theirs as well. So I rang them and they removed it their end, and everything was fine.
Unlike you I could not delete anything as I just locked up. So for anyone finding themselves in the same predicament I suppose it is best to ring your ISP and ask them to remove the hanging emails from your box if that is the case.
Would you like to comment Adept, the Tech guy at Plusnet reckoned it could have been a corrupted header.
Catch