Add a Signature to Every WordPress Email
Appends a short signature, such as your company name, address and support link, to the bottom of every email the site sends, in both plain-text and HTML emails.
Snippet Library
Email snippets for WordPress
Snippets for the emails WordPress sends: change the sender name and address, add a Reply-To header, send HTML email and turn off notifications you do not need. For site owners who want their messages to arrive and look right.
Sign Outgoing Emails With DKIM
Adds a DKIM signature to every email using a private key file and settings stored in wp-config.php, so receiving servers can verify your messages.