The maximum number of messages that a Mailbox can hold is unknown, but the Mailbox will only display the first 50 messages. Any messages over 50 will still exist within the Mailbox, but will not be displayed. To view the messages beyond the first 50, some of the messages will have to be removed (by reading and taking the attachment or by deleting). Once a message has been removed (making a spot available for one of the non-displayed messages), simply close the mail window and reopen the mailbox. (Prior to
patch 2.0.10, you would need to logout or talk to an auctioneer)