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.