The 7-letter word "mailbox" contains thousands of letters in it. The word "mailbox" itself is made up of seven letters, but it is used to receive and contain letters (correspondence) that can number in the thousands when referring to physical mail.

The seven-letter word that may have hundreds of letters in it is "mailbox." The idea here is that a "mailbox" is a container or receptacle for letters, and while the word itself has seven letters, a physical mailbox can hold numerous letters or pieces of mail.