USER name — Returns +OK or –ERR.
PASS string — Returns +OK, usually with a message or –ERR.
QUIT — Closes connection; may not return anything.
STAT — Returns “+OK nn mm”, where nn is the number of messages, and mm is the size of the mailbox.
LIST [msg] — If LIST is given with a message number, it returns “+OK nn mm”, where nn is the number of the message, and mm is the size of the message. Without any argument, LIST returns a line for each message in the format “nn mm”, terminated as usual with a lone dot.
RETR msg — If msg is a valid (not deleted) message number, RETR returns +OK and the entire message (followed by the obligatory dot), or –ERR if msg is invalid.
DELE msg — Marks a message as deleted. Returns +OK or –ERR. The messages are not deleted until the QUIT command is sent, and may be undeleted with RSET.
NOOP — Does nothing and returns +OK.
RSET — Unmarks any messages marked as deleted and returns +OK.
TOP msg n — Retrieves the header of message msg and n lines of the body of the message. Returns +OK or –ERR if msg is invalid.
sumber : unixreview.com