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SMTP is very simple - all 4XX are treated same way - retry later. 5XX is reject.<br />
Most mail operators make it 4XX so that spammer hosts choke from the weight of spam.<br />
Can you chack with netstat - how many connections you have with that IP?
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<div>
Right this moment, none. What's the best way to monitor the number of connections throughout a time interval?
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<div>
Ok, I'll try that out tomorrow.
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I have changed the number of allowed concurrent connections, now I need to check out the logs in the next few days and see if the problem is solved.
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I added my own comment to the solutions because it adds useful details.
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extra_recipient_limit can be 99 as per RFC (just like 2 connections are mandated)
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<div class="content wysiwyg-content">
Hello, lately my hosting server's maillog is recording at least 4-5 of these errors per day:<br />
<br />
/var/log/maillog:Feb &nbsp;7 03:38:03 hostxxxxxxxxxx postfix/smtp[18250]: AC07EFD1F2: host mx.pcpuntodoc.it[62.149.12<wbr />8.157] refused to talk to me: 421 mxcmd05.ad.aruba.it bizsmtp FEgo1s00s2pLL4n01 Too many connections, try later.<br />
<br />
I am on Centos 6.6, Apache and Postfix.<br />
<br />
The server is only used to send mails from the various websites hosted there, such as order mails for e-commerce and so on.<br />
<br />
I am unclear as to whether this error is related to my server or to the server receiving the message (more likely).<br />
It mostly happens with two particular servers on the receiving end, the one listed above and another one, both managed by the Aruba italian service provider.<br />
<br />
Is there anything I can do on my end, or should I simply overlook this?<br />
<br />
Also, does anybody remember which are the error codes (equivalent to 421 in this instance) you receive when your server is blacklisted?<br />
<br />
Thanks.
</div>
</div>


Hello, lately my hosting server's maillog is recording at least 4-5 of these errors per day:

/var/log/maillog:Feb  7 03:38:03 hostxxxxxxxxxx postfix/smtp[18250]: AC07EFD1F2: host mx.pcpuntodoc.it[62.149.128.157] refused to talk to me: 421 mxcmd05.ad.aruba.it bizsmtp FEgo1s00s2pLL4n01 Too many connections, try later.

I am on Centos 6.6, Apache and Postfix.

The server is only used to send mails from the various websites hosted there, such as order mails for e-commerce and so on.

I am unclear as to whether this error is related to my server or to the server receiving the message (more likely).
It mostly happens with two particular servers on the receiving end, the one listed above and another one, both managed by the Aruba italian service provider.

Is there anything I can do on my end, or should I simply overlook this?

Also, does anybody remember which are the error codes (equivalent to 421 in this instance) you receive when your server is blacklisted?

Thanks.

Error 421 on Centos Postfix maillog

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

The Apache HTTP Server is a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards. Typically Apache is run on a Unix-like operating system, but it is available for a wide variety of operating systems, including Linux, Novell NetWare, Mac OS-X and Windows. Released under the Apache License, Apache is open-source software.

Apache Web Server

Linux is a UNIX-like open source operating system with hundreds of distinct distributions, including: Fedora, openSUSE, Ubuntu, Debian, Slackware, Gentoo, CentOS, and Arch Linux. Linux is generally associated with web and database servers, but has become popular in many niche industries and applications.

Linux

Interactions between email servers and clients are governed by email protocols. The three most common email protocols are POP, IMAP and MAPI. Most email software operates under one of these (and many products support more than one).  The correct protocol must be selected, and correctly configured, if you want your email account to work.