One of the IP addresses for Email Security.cloud email delivery servers has been blacklisted by SORBS which is a public block list employed by the intended recipient of the email.
An error can be seen in Track and Trace log.
Error: 554 5.7.1 Service unavailable; Client host [XX.XX.XX.XX]
blocked using dnsbl.sorbs.net; Currently Sending Spam See:
Note: XX.XX.XX.XX is one of the IP addresses of Email Security.cloud delivery servers.
The error indicates that Email Security.cloud attempted to deliver the message to the intended recipient’s server, but the message was rejected by the server, due to the email block list of SORBS. IP addresses for ESS may occasionally get blacklisted due to users who abuse the service by sending spam, or from compromised servers relaying spam through the ESS infrastructure.
Broadcom proactively monitors email block lists and raises a removal request to SORBS in real time, so delivery problems associated with this error is usually temporary.
It will not be necessary to log a support case for requesting a removal to Broadcom. Please contact the sender to resend the mail when you noticed the error mentioned above.