What Is a Self-Mailer? A self-mailer is a type of mail piece that is designed to be sent through the mail without an envelope. It is a self-contained mailer that can be folded, sealed, and addressed by the sender before mailing. There are different types of self-mailers, such as postcards, bi-fold and tri-fold mailers, and booklets.Self-mailers must comply with certain postal regulations to ensure they can be...