I've run into a strange (I think) issue with a person with a Mac, using Mail.app exchanging emails with a person using the Zimbra Web Client. We're in the process of migrating to Zimbra, and the Mac user is still on our old Linux-based, Postfix/POP3 server (not sure if this has any bearing on the problem or not).
The Mac user, who has her message format set to HTML, sent a message with a logo in TIFF format attached to someone using the Zimbra Web Client. The Web Client user received the email no problem. The Web Client user, who is also formatting messages in HTML, then replied to the Mac user with a simple "thank you". When the Mac user got the reply all she got was a message with a 'mime-attachment.tiff' attachment. When the Mac user tried to open this attachment it said the file was corrupt.
I searched bugzilla and found several bugs related to MIME, including one that was concerning Macs. However, I'm not sure this is the same circumstances as any of them.