Unable to send emails via the Roundcube webmail or a PHP script on a Plesk server: encryption needed to use mechanism

Symptoms

• Emails cannot be sent via the Rouncube webmail with the following error message:

  PLESK_INFO: SMTP Error (535): Authentication failed.

• Emails cannot be sent via a PHP script:

  PLESK_INFO: SMTP -> ERROR: AUTH not accepted from server: 535 5.7.8 Error: authentication failed: encryption needed to use mechanism
  SMTP -> FROM SERVER:250 2.0.0 Ok
  
  Fatal error: Uncaught exception 'phpmailerException' with message 'SMTP Error: Could not authenticate.' in

• The following error message appears in /var/log/messages:

  CONFIG_TEXT: postfix/smtpd[21524]: warning: localhost.localdomain[::1]: SASL PLAIN authentication failed: encryption needed to use mechanism

Cause

The current Postfix configuration does not allow the use of mechanisms that transmit unencrypted username and password.

Resolution

1. Connect to a Plesk server via SSH.

2. Open the file /etc/postfix/main.cf in a text editor. In this example, we are using the vi editor:

   # vi /etc/postfix/main.cf

3. Comment out the lines smtpd_sasl_security_options and smtpd_tls_auth_only by placing a # (hash) character at the beginning of each line:

   CONFIG_TEXT: #smtpd_sasl_security_options = noplaintext
   #smtpd_tls_auth_only = yes

4. Save the changes and close the file.

5. Restart Postfix:

   # service postfix restart