Making its appearance again in Mountain Lion is the ability to assign a keyboard shortcut that'll enter an outgoing email account for a message you are composing in Mail. This feature was available in earlier versions of OS X, but was removed in OS X Lion. Now that the shortcut is back in Mountain Lion, it's time to revisit this feature and show you how to set it up. You can use these instructions below to assign the shortcut and use it when composing an email.
Before you get started, you need the name of your email account as it appears in the Mail application. If you don't know these details, then you should open Mail, compose a message and click on the From field to see the available email accounts. The syntax should look something like this:
To set the keyboard shortcut, you must open the Keyboard panel in the System Preferences and click on the Keyboard Shortcuts tab as shown below. You can set shortcuts for a variety of apps from this menu, but we are going to focus on Mail.
Click on "Application Shortcuts" on the left and look for the + icon at the bottom center. Click on this + and select "Mail" from the Application menu. Type in your email address (don't forget the formatting shown above) into the Menu Title and enter your Keyboard Shortcut combo. As noted by Kirk McElhearn in Macworld hints, the keyboard combo should be easy to remember and ideally related to your email account. I used this suggestion in the example below and selected command-option-control and the letter T for my TUAW account. You can click Add when you are done.
The next time you are composing a message, it'll be extremely easy to select the correct email address for the From field. All you have to do is type the shortcut command and Mail will enter the email address you assigned in the shortcut. You can setup shortcuts for a few email address or all of them, if you prefer.