A few days ago, our on-prem Exchange 2016 mail server was blacklisted on about 5 different RBL sites (per mxtoolbox). I reached out to all sites, and requested additional information to determine the root cause. While waiting for their response, what other steps can I perform to track down the root of this evil?
Port 25 is locked down to only allow server access. I also checked the Queue Viewer, but only saw about 21 emails in the queue.
What other steps can be performed?