In the new, the Leopard version of Apple Mail.app they introduced so called Data Detectors. These try to recognize useful data in the mail text to offer special actions on them. Such useful data are for instance addresses, telephone numbers or dates. When you place your cursor over such data in a mail message Mail.app is marking them and offering actions like creating an appointment from an date, large typing telephone numbers or creating addressbook entries from signatures.
But like me most of the people hardly ever use these features. But on the other hand the effects of the Data Detectors disturb selecting the text for copy and paste. Unfortunatly you can’t turn them off via the Mail.app preferences. You have to open the Terminal and type the following magic words to disable them:
defaults write com.apple.mail DisableDataDetectors YES
Now simply restart Mail.app and they’re gone. To reactive the Data Detectors obviously this should help:
defaults write com.apple.mail DisableDataDetectors NO