On another regular day, a mail clients asks the mail server, “I wonder if there were new emails for me today?” And this is their conversation.

Client:    USER hans.hamid
Server:    +OK User accepted
C:    PASS hans.hamid-password
S:    +OK Pass accepted
S: <wait for connection on TCP port 110>
C: <open connection>
S:    +OK POP3 server ready <1896.697170952@ms.domain>
C:    APOP hans.hamid c4c9334bac560ecc979e58001b3e22fb
S:    +OK hans.hamid's maildrop has 2 messages (320 octets)
C:    STAT
S:    +OK 2 320
C:    LIST
S:    +OK 2 messages (320 octets)
S:    1 120
S:    2 200
S:    .
C:    RETREIVE 1
S:    +OK 120 octets
S:    <the POP3 server sends message 1>
S:    .
C:    DELE 1
S:    +OK message 1 deleted
C:    RETREIVE 2
S:    +OK 200 octets
S:    <the POP3 server sends message 2>
C:    QUIT
S:    +OK dewey POP3 server signing off (maildrop empty)
C:  <close connection>
S:  <wait for next connection>