Are there any OpenSourceMailClients? If not, anyone interested in creating one?

Ofcourse there is. Thunderbird from mozilla is open source, even though it isn’t built on cocoa..

Cocoa mail client listed on sf: http://sourceforge.net/projects/ginko

GNUMail.app is GNUStep’s equivalent to Mail.app and it runs on the Mac. http://www.collaboration-world.com/cgi-bin/project/index.cgi?pid=2/