1.2.3 domain spoofing prohibited

SMTP 1.2.3 domain spoofing prohibited This started appearing about 2 weeks ago and since then not receiving emails and unable to send emails. Any thoughts on how I can resolve please?

Domain spoofing, a common form of phishing, occurs when an attacker appears to use a company’s domain to impersonate a company or one of its employees. This can be done by sending emails with false domain names which appear legitimate, or by setting up websites with slightly altered characters that read as correct.

One thing you can check is that you have the correct email address for the account you are sending/receiving through. Most providers do let you use different login and email addresses. So if they are not the same, either by mistake or intention, this could be the cause.