Thanks, Burnout. I'm trying to figure out a way way to automatically duplicate the mail folder every time I shut down Opera Mail, so that if it goes weird when I open it, I've something to switch to. Something new for me to learn...

Interesting about not using low bandwidth with IMAP - done that now. I have POP too and the page link you sent helps but it's quite a rigmarole, so I'll leave that for now. I think the corruption was connected to labels in some way. At the moment it is automatically labelling some emails that it shouldn't. It also shows 7 unread emails under the Labels sections, but there are none to be found. I'm expecting another crash soon...