macosmail.app
OS X Mail – How can I setup multiple e-mail addresses under the same mail account?
I use two addresses with the same account one is me@myworkdomain.com and the other is me@mypersonaldomain.com. On iOS I can add the second address to the mail account configuration and I can send from both.
How is this done on OS X's Apple Mail?
Unfortunately, the answer is no. The hosts file is a static lookup file, and because of this it has no concept of multiple-address per name failover, round-robin, or other features baked into the name resolver.
You can specify multiple names for a given IP address, but not multiple IP addresses mapping back to a single name.
Dare I ask, why can't you access the external interface of the mailserver while internal to your office? We configure all of our clients to use Mail/Jabber externally so they never run into this issue.
Despite conventional wisdom, using the external interface when internally does not "pull all the way across the internet", if configured correctly, of course.
In the Mail.app preferences you can select the outgoing Mail server.
Changing it to your preferred account should be enough.
However, in case this is not enough you could delete the unwanted outgoing servers by editing the SMTP server list:
Best Answer
To add another email address to an account you add it to the email field.
Each address is separated by a comma. ","
But note just because you can add it does not mean you email provider will accept an address from a different provider/domain.
Edit: As of macOS Sierra, the procedure is slightly different. The "Email Address" field is now a dropdown, and you add additional addresses by selecting "Edit Email Addresses..." from this dropdown.