(mailer is a kind of ...) any object that can be used to hold things (especially a large metal boxlike object of standardized dimensions that can be loaded from one form of transport to another)

• container

(... is a kind of mailer ) a mailer for airmail

• airmailer

(mailer is a kind of ...) a public promotion of some product or service

• ad
• advert
• advertisement
• advertising
• advertizement
• advertizing

(mailer is a kind of ...) a human being

Synonyms for mailer in the sense of this definition

• norman mailer

(mailer is an instance of ...) writes (books or stories or articles or the like) professionally (for pay)

• author
• writer