Unable to send email using Horde / Roundcube / mail software on Plesk server: Error connecting to SMTP server

Symptoms

• The following error message appears when mail is sent in Horde / Roundcube / mail software on Plesk server:

  PLESK_INFO: Warning: There was an error sending your message: Error connecting to SMTP server.

• In Horde log /var/log/psa-horde/psa-horde.log, the following error can be found:

  CONFIG_TEXT: ERR: HORDE [imp] Error connecting to SMTP server. [111] Connection refused [pid 6716 on line 1160 of "/usr/share/psa-horde/imp/lib/Compose.php"]

• The following error message appears when mail is sent in Roundcube / mail software:

  PLESK_INFO: SMTP Error (-1): Connection to server failed

• Postfix is used as mail service.

• Postfix service is in stopped status:

  # service postfix status
  Redirecting to /bin/systemctl status postfix.service
  ● postfix.service - Postfix Mail Transport Agent
  Loaded: loaded (/usr/lib/systemd/system/postfix.service; enabled; vendor preset: disabled)
  Active: inactive (dead)
  server-hostname systemd[1]: Failed to start Postfix Mail Transport Agent.
  server-hostname systemd[1]: Unit postfix.service entered failed state.
  server-hostname systemd[1]: postfix.service failed.
  

Cause

SMTP Server Postfix is not running and thus cannot process mails.

Resolution

1. Connect to the server via SSH.
2. Start the service SMTP Server (Postfix):

   # service postfix start