Incoming mail from 1&1 ionos server 550 Email blocked by

An email sent from an ionos mail server fails to deliver, with the following message:

550 Email blocked by

I entered the IP addresses in the failed delivery messages into the tool at (there were two IPs in this email, a gmail one and an ionos one) both IPs come up as not blacklisted.

Any ideas what could be causing failed delivery and how it could resolved? This company has emails on both mxroute and ionos so it is problematic for them to not be delivered (if that is the issue)

What was the IP? Feel free to send it privately (

Edit: Nevermind, found it. I cleared the block and narrowed it down to the actual spammer that I based the RBL entry on:

Thanks for your help, it’s working now!