Mail.app SMTP Question

I am back to the MAC side after a few years. (Well actually quite a few)

I have set up three POP3 accounts in Mail. When I try to send mail I occasionally get the following message.

The server response was: 5.7.1 <webmaster@macrumors.com>... we do not relay <m@charter.net>

Use the pop-up menu below to try a different outgoing mail server. All messages will use this server until you quit Mail or change your network settings.

In the Windows world with Outlook I would check the "require logon using Secure Password Authentication (SPA)" checkbook and then under "more settings" Outgoing Server I would check "My outgoing server (SMTP) requires authentication. Use same settings as my incoming mail server.

I haven't quite figured out how to set something like this in Mail. Any help is greatly appreciated.

"Proud owner of two MacBook Pros"