Did you give the "new" Opera a try in regards to your large email "collection"? I had similar issues in the past, but the new version is surprisingly good in handling large repositories. Just as an idea ;)
True! I did use that for 2-3 months over the summer (version 11? 11.5?) , and it worked reasonably well. I eventually moved away, because it had a bunch of locking pauses.. I'm not sure what caused them. My guess is that when checking for mail in account A, account B,C,D would lock up?