Howdy, Saw something similar recently myself. In a nutshell the receiving party is verifying the sender address by connecting back to your sender domain's mail server (e.g. the Sophos XG acting as the domain's mail server) and attempting to deliver a null message to the sender address. The problem is that the receiving party is using an outbound server that's most likely in in the UCEPROTECT-L1 blacklist (it was in my case). The workaround is to either remove the dnsbl-1.uceprotect.net entry from the Standard RBL Services list (preferable - this entry has caused more grief than good over the years), or create a mail scanning exception to skip RBL checks for the outbound server IP in the UCEPROTECT-L1 blacklist. Assuming that the UCEPROTECT-L1 blacklist is the problem. You will also need to use the Log Viewer, go to Firewall and filter on inbound SMTP connections, as the Sender IP address won't show in either the Mail Logs view or in the Mail section of Log Viewer. I had to make a note of the timestamps in the Mail Logs view, then correlate them in Log Viewer. Which is a reminder that Log Viewer is still pretty bad at providing transparency of the raw log files stored on the XG. Hope this helps work out your specific issue with the 'Sender IP address is blacklisted' problem.
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