Help Mail appeared to be spam or forged

Error message: “Recipient address rejected: Mail appeared to be SPAM or forged”

Suppose someone wants to send you a mail to your mailbox address.
It may happen that the mail does not reach you, and instead the sender always receives the following error message:

Remote host said: 550 : Recipient address rejected:
Mail appeared to be SPAM or forged. Ask your Mail/DNS administrator
to correct HELO and DNS MX settings or to get removed from DNSBLs;

The problem here lies in the configuration of the mail server that delivers the mail to us. We check its DNS name using its IP address. We also check the value it passes to our server as HELO – as defined in the Simple Mail Transfer Protocol (SMTP).

If these two do not match, we reject the mail. And for good reason: a mismatch between DNS and HELO names often occurs on servers that are misused for sending spam mails.

According to RFC 2821 (the current definition of the SMTP protocol), the HELO is used to identify the client (i.e. the mail server that wants to send a mail to our server) and should therefore correspond to its Reverse DNS Lookup (in which the domain name stored in the DNS is determined inversely from the IP address).

In situations where the client cannot specify a meaningful HELO (for example, due to a dynamic IP address), RFC 2821 recommends using the IP address in the HELO instead.

Cause of the error message

The client is probably simply specifying the sender domain of the mail as HELO instead of its own host name.
Although this is not prohibited, it is neither a sensible configuration nor does it comply with the SHOULD requirements of RFC 2821.

Solution to the problem

The sender of the mail that did not reach you must contact their mail provider (and ask them, for example, to use HELO in accordance with the standard).
Neither you nor we can resolve this issue.