Issue SMTP / connection dropped for one sender

[Erwan]

Server operating system version

CentOS 6.10

Plesk version and microupdate number

18.0.31

Hi,

We have a problem with receiving emails on one server.

A sender receives the following message when sending emails:
Message delivery to the host X.X.X.X failed while delivering to the remote domaine 'MYDOMAIN.COM' for the followinf reason: The connection was dropped by the remote host.

But no problem with other senders. The messages are correctly received.

The server is old. It uses Courier-IMAP / Qmail.

Nothing in /var/log/maillog about the sender with the problem.
In /var/log/messages, we have:
FAIL: smtp per_source_limit from=::ffff:xx.xx.xx.xx

xx.xx.xx.xx is the IP from the sender with the problem.

So, i've updated /etc/xinetd.conf:
# Define access restriction defaults
#
# no_access =
# only_from =
# max_load = 0
cps = 50 10
instances = 50
per_source = 1000

The value was pers_course = 10 before. Is this the solution? anything else to do?

Thank you.

Erwan

 

[Peter Debik]

This could also point to a blacklist ban of the sender's IP.

 

[Erwan]

Hello Peter,

That's what I thought at first, but no sender IP at fail2ban or firewall.

 

[Peter Debik]

Do you have any blacklists (DNSBL) configured in your mail server settings?

 

[Erwan]

yes, xbl.spamhaus.org;sbl.spamhaus.org but no problem with sender ips too.

Messages are starting to arrive so the problem seems to be solved with the modification.

Last question:
Do you know the interval / range for xinet "per_source_limit"?
Concurrent access, per hour...?

 

[Peter Debik]

It can be anything up to "UNLIMITED". For details please see xinetd.conf(5) - Linux man page
But: Setting too high values can lead to excessive overload of a server when too many requests shall be handled which can make a server unresponsive. There is no clear rule how much is "too many". 1000 sounds like a lot already.