Email rejection with the error: "Client host rejected: cannot find your hostname"

Symptoms

When sending an email, a bounce-back may have the following error:

Feb 5 05:33:25 mail postfix/smtpd[15726:] NOQUEUE: reject: RCPT from unknown[192.0.2.0]: 
450 4.7.25 Client host rejected: cannot find your hostname, [192.0.2.0]; 
from=<sendingemail@exampledomain.tld> to=<recievingemail@otherdomain.tld> 
proto=ESMTP helo=<hostname.maindomain.tld>

Description 

Checks against the hostname are needed to ensure that the hostname is properly resolving to the server. Here are the commands needed to perform this check:

1 . Check the hostname resolves from a workstation other than the server:

dig hostname.maindomain.tld +short

The proper output will be the server's IP address:

cptech@cPanel.net [~] # dig hostname.maindomain.tld +short
192.0.2.0
cptech@cPanel.net [~] #

2 . Make sure the IP from the above command is bound to the server by running the following on the server:

[Non-NAT'ed Environment]

ifconfig | grep 192.0.2.0

The proper output if the server is not in a NAT'ed environment:

[root@server ~]cPs# ifconfig | grep 192.0.2.0
    inet 192.0.2.0 netmask 255.255.255.255 broadcast 192.0.2.0
[root@server ~]cPs# 

[NAT'ed Environment]

grep 192.0.2.0 /var/cpanel/cpnat

The proper output if the server is in a NAT'ed environment:

[root@server ~]cPs# grep 192.0.2.0 /var/cpanel/cpnat
10.0.0.0 192.0.2.0
[root@server ~]cPs#

3 . Check that the server is responding to mail requests by running the following on a different workstation other than the server:

telnet hostname.maindomain.tld 25

The proper output with include the hostname or the main domain for the account in the SMTP banner:

cptech@cPanel.net [~] # telnet hostname.maindomain.tld 25
Trying 192.0.2.0...
Connected to hostname.maindomain.tld.
Escape character is '^]'.
220-hostname.maindomain.tld ESMTP Exim 4.93 #2 Tue, 16 Feb 2021 09:47:02 -0600 
220-We do not authorize the use of this system to transport unsolicited, 
220 and/or bulk e-mail.

** SNIPPED **

4 . Use the cPanel provided "cpdig" script on the server to verify that the hostname passes the validation check:

/scripts/cpdig hostname.maindomain.tld A

The proper output will be the server's IP address:

[root@server ~]cPs# /scripts/cpdig hostname.maindomain.tld A
192.0.2.0
[root@server ~]cPs#

5 . Check that the hostname is considered valid with the script on your server:

/scripts/check_valid_server_hostname

The proper output will result in "OK":

[root@server ~]cPs# /scripts/check_valid_server_hostname
OK
[root@server ~]cPs#

Workaround

If all of the commands provided show the correct IP, hostname, or result in "OK," then the issue may have been temporary. Please attempt to send the email again in a few moments. 

If the email still does not send, please reach out to the receiving email provider and work with the email provider to resolve the issue. The issue may be a misconfiguration on their email server(s).