When using Mac Mail, in Yosemite you may want to add an email signature to your emails. To do this, you must create a signature in preferences.
Choose Mail > Preferences, then click Signatures.
In the left column, select the email account you want to use the signature for, then click .
You can drag signatures from one account to another, or to and from All Signatures. If you create a signature in All Signatures, you must drag it to an account to use it.
In the middle column, type a name for the signature.
The name appears in the Signature pop-up menu in the message header when you write a message.
In the right column (the preview), create your signature.
You can use the Edit or Format menus in Mail to change the font or layout, turn text into links, or check spelling. To add an image, drag the file into the preview.