Article ID: 1390, created on Oct 6, 2008, last review on Nov 27, 2014

  • Applies to:
  • Plesk 8.x for Linux/Unix


You can check if SMTP authorization works using a telnet session to to your mail server on port 25. Let us use the example user name "user@domain.tld" and "password" as the password.

# telnet 25
Connected to domain.tld (
Escape character is '^]'.
220 domain.tld ESMTP
auth login
334 VXNlcm5hbWU6
334 UGFzc3dvcmQ6
235 go ahead

Let us see this process step-by-step:

- Run the telnet command from the shell using the Plesk server IP and SMTP port number as parameters. Note that you should run the command from a remote box. If you run telnet from the Plesk server itself, you don't require authorization because the localhost is added in Plesk to the whitelist by default.

- You will receive the Qmail greeting "220 domain.tld ESMTP."

- Write "auth login" to telnet.

- Server answers with "334 VXNlcm5hbWU6"; it is the base64-encoded string "344 Username:"

- Input the base64-encoded user name. Use any public base64 encoder, e.g.:
        Base 64 Encoder

- Server replies with "334 UGFzc3dvcmQ6"; it is the base64 encoded string "344 Password:"

- Input the base64-encoded password.

- If you get "235 go ahead" after you put in the base64 encoded password, then SMTP authorization on the server works fine.

- If you get "535 auth failure," it means that the mail user name and password do not match or there is some misconfiguration on the server. In this case, try to change the password for this mailname through Plesk CP and see if it helps. If not, you can use the "mchk" utility to rebuild Qmail configuration using article 944 and check Plesk SMTP service functioning as described in article 1387.

- If you see the "503 you are already authenticated" message after entering the "auth login" command, there are two possible reasons:
a) You were already authorized via POP3 authorization.
b) Your IP is listed in the whitelist, and you don't need to be authenticated in this case.

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