The term 'MAILS OFF' is composed of two words, 'POSTS' and 'SENDS'. 'POSTS' is a five-letter word meaning to send something through the mail or to deliver it to a specific place. 'SENDS' is also a five-letter word meaning to cause something to go from one place to another, usually by means of a messenger or courier service. Together, the two words form the phrase 'MAILS OFF', which is used to describe the act of sending something through the mail.