Mail.app is a really good and efficient email client for the Mac. But it has some lacks and annoying nuts. One of those is the fact that since Mac OS X 10.6, when you copy an email address, you also copy the name at the same time.
Close Mail.app and type this in a terminal window :
defaults write com.apple.mail AddressesIncludeNameOnPasteboard -bool NO
You can revert it back by typing this command :
defaults delete com.apple.mail AddressesIncludeNameOnPasteboard