In all versions of Mac OS X, Mail automatically works with some mail servers to verify the validity of your email message's recipients. It does this if the mail server has been configured to perform verification. If the server identifies any recipient's address as invalid or unrecognized, this alert will appear, along with an identification of which addressee is not recognized:
"The mail could not be sent, but is saved in your Outbox until it can be sent."
If this happens, edit the message that's in the Mail outbox: Remove invalid recipients, then try to send the message again.