An email sent from an ionos mail server fails to deliver, with the following message:
550 Email blocked by bl.mxrbl.com
I entered the IP addresses in the failed delivery messages into the tool at http://mxrbl.com/lookup.php (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)