How to Add Signatures to Emails
To automatically add a signature to emails that you send from your Macintosh computer (Mail app), you must create a signature template in preferences.
Open the preferences window by selecting Mail from the top menu and selecting Preferences.
Select the Signatures tab.
On the Signatures tab select your email address in the left column, then press the plus sign (+) in the middle column. You may give your new signature a custom name. Finally create your signature in the right hand column.
Additionally you may select a default signature with Choose Signature. And you may also change the location of the signature in your messages by checking or unchecking the box next to Place signature above the quoted text.