Cannot send email: Too many concurrent SMTP connections

Symptoms

Cannot send email. The following error appears:

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

Postfix is used as a mail server.

Cause

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

Resolution

Note: if you don't have root access to Plesk server via SSH, contact your hosting provider regarding the issue

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. Apply the configuration change:

# 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