Post Office Protocol 3 (POP3) commands:

• USER - Username to use for authentication
• PASS - Password to use for authentication
• STAT - Mailbox statistics (+OK $TOTAL_MSGS $MBOX_SIZE_BYTES)
• LIST - List messages ($MSG_NUMBER $MSG_SIZE_BYTES)
• RETR - Retrieve message $MESSAGE_NUMBER
• QUIT - Close connection

There are other commands, but the above are enough to pull messages.