Cannot send email in Plesk for Linux: Too many concurrent SMTP connections

Symptoms

1. Cannot send email:

   CONFIG_TEXT: Too many concurrent SMTP connections; please try again later

2. Postfix is used as mail server. It can be found in Tools & Settings > Services Management.

Cause

Remote mail servers restrict SMTP connections due to exceeded concurrent connections count limit.

Resolution

1. Login to Plesk via SSH
2. Back up Plesk configuration:

   # cp /etc/postfix/main.cf /etc/postfix main.cf.bak

3. Add the following parameter to /etc/postfix/main.cf:

   # PARALLEL DELIVERY TO THE SAME DESTINATION
   smtp_destination_concurrency_limit = 20

4. Reload Postfix:

   # service postfix reload

5. Try to send emails to reproduce the issue. If it still appears, reduce the smtp_destination_concurrency_limit value. If it does not appear, increase the value in order to minimize affecting mail server performance.

Additional information

Tuning the number of simultaneous deliveries