Given a header like this: Subject: =?UTF-8?Q?[MTD]=20CAF=C3=89=20NAND:=20Add=20'slowtiming'=20parameter, =20default=20usedma=20and=20checkecc=20on?= ... Evolution fails to decode it because of the whitespace in it (the newline). Note that this is Evolution's "raw" message view, and may be whitespace-corrupted by Evolution (bug 83257, January 2003). But it does seem to be the line break which upsets Evolution. AFAICT Evolution doesn't handle =?UTF-8?B?...?= format at all, either.
The =UTF8?Q?...?= bug with the newline in seems to be a mailman bug (bug 217667) -- I think RFC2047 exonerates Evolution on that score, although perhaps an argument could be made for being liberal in what we accept. I was sure that I'd also seen Evolution consistently fail to decode headers of the form =?UTF-8?B?SGVsbG8=?= too, but upon testing it seems to work, so either it's fixed in FC6 or I was imagining it -- or it was just badly formed in the spam I was looking at. Closing this bug accordingly.