@stavros mitchell
What version of your mail server?
This email system failed to validate the source domain (reverse DNS lookup) This security issue is used often to compromise users from lookalike domains, your inbound servers do not check if the domain is valid before accepting the email. This security issue is used often to compromise users, steal data, and manipulate internal affairs.
If you cannot send email from your mail server to target mail server. You can try to contact you ISP to add a reverse DNS lookup for your mail server. By the way, if your mail server doesn't have SPF record, try to add it together. Lack of SPF records will be rejected by many mail servers.
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