Can't send outgoing messages in Mac Mail
As stated before, all this means is your outgoing server credentials are incorrect in the Mail App. 

In order to check the outgoing server's credentials go to  Mail menu > Preferences > Accounts > find your Gmail account. Click on your gmail account then under "outgoing server" check what server is chosen by default then click "edit smtp server list" at bottom.

Once there double check all the server info. Make sure it has your email spelled correct and password correct, make sure authentication is set to "password" then make sure you use SSL and use port 465 or 587.