Unsolicited bulk e-mail (UBE).

[NetworkExpertise]

Why do my customers get the message below when forwarding and email with a link in it to someone? They are using PORT 587 for SMTP too!

> Our internal reference code for your message is 26618-06/PyXvg8CcMrKh
>
> The message carried your return address, so it was either a genuine
> mail from you, or a sender address was faked and your e-mail address
> abused by third party, in which case we apologize for undesired notification.
>
> We do try to minimize backscatter for more prominent cases of UBE and
> for infected mail, but for less obvious cases of UBE some balance
> between losing genuine mail and sending undesired backscatter is
> sought, and there can be some collateral damage on both sides.
>