Improper use of SMTP command pipelining
I have all my servers on AlmaLinux 8, and running the same exim configuration.
There is only one that is getting responses "Improper use of SMTP command pipelining" sending to different domains.
I already disabled pipelining on basic editor, with no changes.
After that, I resetted to default, configured everything again. And disabled again. Still failing.
The pipelining references I see on advanced configuration are this:
remote_smtp:
driver = smtp
# Disable pipelining for all hosts
hosts_avoid_pipelining = *
interface = <; %CPANEL-smtp-interface%
helo_data = %CPANEL-helo-data%
hosts_try_chunking = 198.51.100.1
message_linelength_limit = 2048
dkim_remote_smtp:
driver = smtp
# Disable pipelining for all hosts
hosts_avoid_pipelining = *
interface = <; %CPANEL-smtp-interface%
helo_data = %CPANEL-helo-data%
dkim_domain = ${perl{get_dkim_domain}}
dkim_selector = default
dkim_private_key = "/var/cpanel/domain_keys/private/${dkim_domain}"
dkim_canon = relaxed
hosts_try_chunking = 198.51.100.1
message_linelength_limit = 2048
.ifdef ARCSIGNING
arc_sign = $primary_hostname:default:/var/cpanel/domain_keys/private/$primary_hostname:default
.endif
I have not this problem on other server, even with pipelining enabled.
Any ideas?
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Hey there! Usually this error indicates some type of networking issue outside of the server. As a test, you can try this from the command line:
nc -v mx1.cpanel.net 25
That should give you the following output:
Ncat: Version 7.92 ( https://nmap.org/ncat )
Ncat: Connected to 208.74.121.68:25.Once that shows up it will pause and you can type in the following to introduce yourself (the actual hostname doesn't matter for this test - you can use the test server name just as it's formatted below):
ehlo your.servername.com
and then you'll get the SMTP transaction output. If you still see the "Improper use of SMTP command pipelining" error you can be confident it isn't part of Exim or your email server settings since it's happening elsewhere.
If that does reproduce the issue you'll want to speak with your host to have them test things on their end.
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Thank you, this is the result. Data anonymized.
[root@iapetus ~]# nc -v mx1.cpanel.net 25
Ncat: Version 7.92 ( https://nmap.org/ncat )
Ncat: Connected to 208.74.121.68:25.
ehlo xxxxx.yyyyy.com
220-mx1.cpanel.net ESMTP Exim 4.98.1 #2 Mon, 21 Jul 2025 15:00:32 -0500
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
250-mx1.cpanel.net Hello xxxxx.yyyyy.com [x.y.z.x]
250-SIZE 125829120
250-LIMITS MAILMAX=1000 RCPTMAX=50000
250-8BITMIME
250-PIPECONNECT
250 HELP
Should I continue sending the email?0 -
No, the error would have shown up in those headers.
Does this error appear when you send a message in Webmail?
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Thank you!
I have confirmed failed emails sent from Oultook and from a hosted system (Moodle). Not from webmail.
Actually it seems a bit random. The moodle user sent today near 100 emails to a unique domain, and only 8 failed (with pipeline error).0 -
I'm not sure then, especially if it's intermittent. It still could be ratelimiting or something else happening with your host, but it doesn't seem to be anything on the Exim or cPanel side from the data we have so far.
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I am suspecting on datacenter network (namecheap). They have some anti spam protections that make some problems from time to time.
Thank you, I will elevate this to support, and if it is needed, they will elevate to cpanel.0 -
I'd be interested to know what you find, so report back when you can!
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