In Addition to clarify my first post FYI:
You could have used another option other than creating another SMTP server in Mail. In my first post, note in the image (in the "Send As section") I have added my att.net account. The reason for this is I can use the gmail server to send messages from my att.net account. At one time, the att servers were always down and I used the gmail server to send my messages.
As I said, the default in GMail when sending a message from a different account throught it is that gmail changes the address "sent from" to the gmail address it is sent through. By adding the att.net account to it and creating the right settings, it sends the messages through as the correct att.net address and does not change it to the gmail address.
Since you only had the one SMTP address in the list, the user account and password used was the default sending account. When you sent from the other gmail account, it changed the address when it hit the server to the default and that's what the reciever got. That's why I asked if you had the other account set to it. You would have had to check what server settings you had in Mail to know which gmail account to look at though. It would have been the one with the user name and password set up in mail. Then you could have added the other gmail account to it at the website settings so it would retain the correct "from address".
This may help clear up what was probably happening to you.