Mail (Mountain Lion): Use signatures
You can create prepared text, called a “signature,” to add to messages before you send them. For example, you might want to add your phone number at the end of your messages.
Depending on how a recipient’s mail server handles signatures, your signature may appear as an attachment or image to your recipients.
If you use iCloud Documents & Data, your signatures are available on your other Mac computers that use Documents & Data and have OS X Mountain Lion installed.
Create a signature
Choose Mail > Preferences, and then click Signatures.
You can create multiple signatures for each Mail account. For more information about options, click the Help button in the Signatures pane.
Add a signature to a message
In a message you’re writing, choose a signature from the Signature pop-up menu.
You may need to customize the header to see the Signature pop-up menu.
If a signature isn’t in the pop-up menu, check in Mail Signatures preferences whether the signature is included in those for the account. If it’s not, click All Signatures, select a signature in the middle column, and then drag it onto an account in the left column.