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:
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.