Knowledge Base

Unable to receive/send emails in Plesk with submission port (587) enabled: Connection concurrency limit exceeded

IMAP accounts are failing to sync when using the Outlook mail client.
The Postfix service stops sending/receiving emails on port 587.
The following error can be found on /var/log/maillog:

postfix/smtpd[12664]: warning: Connection concurrency limit exceeded: 51 from unknown [203.0.113.1] for service smtp

The maximum number of connections that an SMTP client may make simultaneously is exceeded (default is 50).

Either of the following resolutions can be applied:

Connect to the server using SSH
Execute the command below to increase the maximum number of connections that an SMTP client may make simultaneously:

# postconf smtpd_client_connection_count_limit=100

Note: This is a sample value, it needs to be changed according to the server specifics.

Log into Plesk
Go to Plesk > Tools & Settings > IP Address Banning (Fail2Ban) >Jails > Add Jail
Add the following:

Name:

postfix-concurrency

Filter:

postfix-sasl -Postfix SMTP and SASL authentication failures

Action:

iptables-multiport[name="postfix-concurrency", port="smtp,smtps,submission" protocol="tcp"]

Log Path:

/var/log/maillog

Ban Period:

600

The maximum number of failed login attempts:

5
Go to Tools & Settings > IP Address Banning (Fail2Ban) >Jails > Manage Filters > Add Filter
Add the following:

Name:

postfix-concurrency

Content:

[Definition]
failregex = warning: Connection concurrency limit exceeded: [0-9]+ from S+[<HOST>] for service S+$
ignoreregex =
Connect to the server using SSH
Verify the filter functionality by running the following command:

# fail2ban-regex /var/log/maillog /etc/fail2ban/filter.d/postfix-concurrency.local
If the configuration is correct you will get a result similar to the one below:

Running tests
Use failregex filter file : postfix-concurrency, basedir: /etc/fail2ban
Use log file : /var/log/maillog
Use encoding : UTF-8
Results
Failregex: 0 total
Ignoreregex: 0 total