This command is implemented to allow clients to check the server for RFC 1939 compliance. This method has no other purpose, and is called automatically. This POP3 class already implements this checking. Defaults to headers only. This method returns an array where each line of the message is an array element.
Returns undef on failure.
If no message is specified, this method returns an associative array of undeleted message numbers as the keys, and the associated message size as the element. Array index zero will contain the total number of undeleted messages in the mailbox. Valid message numbers begin at array index one. Similar to TOP , this sends the POP3 RETR command. This method returns the contents of the specified message as an array, with each line of the message being an array element.