Creating a custom email signature will significantly reduce the time it takes to create and send your emails as well as help meet email sending guidelines. Your email signature can include your logo, company name, title, phone, website, street address and social media links.
You can set your signature to show by default on all new emails you create and simply un-check the "use signature" box if you choose to have a different signature for that email. See the email layouts tutorial for more information on how to create other types of re-usable signatures. See the eCards and Email Newsletters tutorial for how to further customize your email signature on each email you create.