It might be because it's searching using the IMAP server, for some reason those can't often deal with non-ASCII characters. Searching with diacritics seems to work for me in locally-downloaded folders.
Well, wasn’t email designed in a time when the only supported standards were everyone’s form of Extended ASCII? So it would make sense that IMAP only supports ASCII. Heck, attachments are encoded in some base## form before being sent because the entire standard is based on text.
Here I like to quote my favorite penguin:"don't give me excuses, give me results". MS products somehow can do it, and it is a big showstopper dor adaptation in (a lot of) countries with special characters if you cannot search for the product name/customer name/project. (But maybe I remember wronfly and MS also cannot do with IMAP, so exchange it is)
