Unable to send email via Roundcube webmail: SMTP Error (454): Authentication failed

Symptoms

• In an attempt to send an email message via the Roundcube webmail on Plesk server, the following error is shown:

  CONFIG_TEXT: SMTP Error (454): Authentication failed.

• In an attempt to send an email message via the Horder webmail, it fails with the error below:
  

  CONFIG_TEXT: Server Denied Authentication

•  The messages like below can be found in /var/log/maillog file:

  CONFIG_TEXT: warning: connect to Milter service inet:127.0.0.1:12768: Connection refused
  NOQUEUE: milter-reject: CONNECT from example.com[203.0.113.2]: 451 4.7.1 Service unavailable - try again later; proto=SMTP

Cause

The pc-remote service is stopped.

Resolution

1. Log into Plesk.
2. Go to Tools & Settings > Scheduled Tasks > Add Task.
3. Perform the following:
   • Select Task Type as Run a command
   • In the Command * field paste the line below:
     

     CONFIG_TEXT: /usr/sbin/service pc-remote start

   • Select root from the drop-down menu System user 
     

     Click on the screenshot below to expand
      

4. Press the Run Now button to start the pc-remote service.