This answer will explain what messages similar to 'ATLAS_RBL [...] is listed at pbl.spamhaus.org' means, and how to stop them being sent.
Email blocking problems
Occasionally, someone who has sent you an email may receive a message saying that a mail they sent you hasn't been delivered. The email they sent has been blocked, and the full message is:
ATLAS_RBL 18.104.22.168 is listed at pbl.spamhaus.org. Contact tech support enclosing a copy of this error message on email address.
The message will contain a full email address in place of email address. There may also be another address in place of pbl.spamhaus.org.
Only the sender will receive the message. You will not receive notification that an email to you has been blocked and you will be able to send and receive other messages as normal.
The message is part of our effort to block spam sent to email addresses hosted on 123-reg.
We check the IP address of the server each email has been sent from against several lists of addresses, one of which is maintained by The Spamhaus Project. If the IP address is on any of these lists then we block the email.
Although this stops lots of spam getting through, it does mean legitimate messages occasionally get blocked too. If this is happening to someone trying to send you email, it’s easy for them to stop it occurring in the future.
The person who sent you the email should forward the message they received to the address shown in it. We'll put the relevant IP address onto our list of allowed addresses, so future mails from that person come through to your inbox.
If the message mentions spamhaus.org, they can also enter the IP address in the message (e.g. 22.214.171.124) into the Lookup IP box on The Spamhaus Project". This will remove the IP address from the Spamhaus list and should ensure that other email providers using the list stop blocking messages from that person.