Our advanced spam filtering solution sometimes falsely detects unsolicited mail that was genuine. There are many reasons for this, but usually, it is down to questionable content, a lot of links, images, or questionable attachments.
Rather than just block these mails by default, the sender is given an opportunity to re-send the mail should it be genuine and they wish to force it through.
If someone that is sending you emails receives a bounce message that says their mail delivery failed, they should follow the instructions in the body of the mail they receive:
This 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(es) failed:
emailaddress@somedomain.com
host somedomain.com [IP_ADDRESS]
SMTP error from remote mail server after end of data:
554 5.7.1 Your email appears to be unsolicited and was not accepted by
somedomain.com . Please try using an alternative email or if you want your
email accepted resend it with the code A2kzRlrsNE appended to your email
subject, thank you.
What this message in bold above is advising the sender to do is forward the original mail with the subject intact, but ADD the code A2kzRlrsNE to the end of the subject. This code will change for every single mail.
The idea of this is to thwart even the most determined spammers, because usually they use automated systems and are not able to pick up these sort of responses.
