IPA: /ˈmeɪlər/
KK: /ˈmeɪlər/
A person or thing that sends or prepares mail, or a container used to hold items for mailing.
The mailer arrived at the post office with a stack of letters to send.
Mailer → It is formed from "mail" (from Old French *male*, meaning a bag or pouch) and "-er" (meaning a person who). A mailer is a person or thing that sends or delivers mail.
Think of a person who sends or delivers mail — the 'mail' refers to the bag or pouch, and '-er' indicates a person doing the action.