I don't think Green Mail supports IMAP. I'll have to double check that to be sure. Anyway, I did find Mail Drop. But it hasn't been worked on in years. And the last release (2.0d which is author said was an alpha,) is missing several features.

Mulberry also supports IMAP, but I've yet to test it, have to get a OS 8/9 version off a mac addict CD since I can't find any online.