unexplained mail delivery failure

For severasl weeks I've been receiving regular emails entitled "Mail delivery failed: returning message to sender" which I don't understand. The message reads:

"A message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address failed: myaddress@btinternet.com.

SMTP error from remote server after MAIL command:

host: mx.bt.lon5.cpcloud.co.uk

IP FOUND IN DNSRBL (please check for IP address listing on common DNS RBL services)

... The header of the original message is following ..."

________________________________________________________________________________ ______________________________________________

The rest of the message means nothing to me except for the fact that I can see that it is associated with an organisation that regularly communicates with me. It also includes the statement "Mail was not recognised as spam".

It is as if someone has sent me an email which automatically generated a reply which failed.

Could this be due to the fact that the address used by my correspondents (myaddress@mail.com) is different from my primary address (myaddress@btinternet.com). Mail.com forwards my emails to btinternet.

Can anyhone explain what is happening?

😕