It seems strange that so many people are having so much trouble; I saw where the Internal Error happened, but as long as an instance of Modest was open, it worked fine. Now with W12, it seems to work fine entirely. I'm using GMail IMAP, SSL IMAP4S, and having no trouble.

And for people who think that fat scrolbars on the default client would make it fine, maybe, if you don't use GMail at least, it would be. But I used it a little before, and I think Modest is much improved. It's beta, of course, and so there are some troubles, but it's already better than the built-in one and improving (in stability) despite the odd glitch along the way.