How do I address MailPlus security log 450 4.7.25: Client host rejected?

Symptoms

In MailPlus Server > Auditing > Log, you received the following message:

450 4.7.25 Client host rejected: cannot find your hostname, [IP address]

MailPlus Server rejected the SMTP connection because the hostname does not match up with the IP address in the HELO command.

Try the following methods to troubleshoot:

Ask the sender to check the mapping between the hostname and IP address in DNS records.
Use MxToolbox to check if the mail server' IP address belongs to the sender domain.
Add the mail server's IP address to MailPlus Server > Mail Delivery > Security > Block/Allow List.
Turn off the hostname check by disabling the following option:
- Reject unknown client hostnames in MailPlus Server > Mail Delivery > Security

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