Only a few POP3 servers currently allow an unencrypted connection between a POP3 client and a POP3 server. The easiest way for such a case is to establish a TCP connection to a POP3 server via the client service of the 'telnet' programme:

[1] hans@linux:~$ telnet gbbuch.de 110
[2] Trying 109.237.140.40...
[3] Connected to gbbuch.de.
[4] Escape character is '^]'.
[5] +OK AlfaHosting GmbH – ready.
[6] USER meinUSERNAME
[7] +OK
[8] PASS meinPASSWORT
[9] +OK Logged in.
[10] LIST
[11] +OK 2 messages:
[12] 1 2593
[13] 2 2833
[14] .
[15] QUIT
[16] +OK Logging out.
[17] Connection closed by foreign host.
[18] hans@linux:~$

Comment:

• In line 1, a TCP/IP connection to the POP3 server 'gbbuch.de' is established on port 110 - the standard port for POP3 - and confirmed in line 3.
• Then, in lines 6 and 8, the user is identified at the POP3 server.
• With 'USER myUsername' the username (line 6) for the corresponding email account is passed and sent in plain text.
• If the user is known on the POP3 server, you can send the password - again in plain text (!) - with 'PASS meinPasswort'.
• With the command LIST in line 10 you ask for a list of the e-mails stored on the POP3 server and here you get a list with two lines, each line containing a (consecutive) number and the size of the e-mail (unit byte).
• The connection between the telnet client and the POP3 server is closed with the command 'QUIT'.

Here → https://de.wikipedia.org/wiki/Telnet you get a short description of the console programme telnet.

The following example impressively demonstrates the reactions of a POP3 server requesting an encrypted connection between POP3 client and POP3 server - which can be considered standard for some years now:

hans@linux:~$ telnet mx.freenet.de 110
Trying 2001:748:100:40::8:111...
Connected to mx.freenet.de.
Escape character is '^]'.
+OK <8386.1448878562@mx.freenet.de>
USER meinUSERNAME
+OK user ok
PASS meinPASSWORT
-ERR permission denied
Connection closed by foreign host.
hans@linux:~$

In the next chapter you will learn how to establish and use an encrypted TCP/IP connection between a POP3 client and a POP3 server in the Linux console.