There was an issue earlier today with one of the RBLs we use listing non-spam URLs. This resulted in issues where some clients’ outgoing messages were being blocked with a message from Exchange Defender saying the message contained a Spam URL.
That issue was resolved but there was a problem in the process that the RBL used, which made it so that it timed out and didn’t function at all, causing sporadic blocks in messaging inbound. This is not on our servers, nor on the recipient or sender MTAs.
The issue is resolved completely now and mail flow should be back to normal. You may still get complaints for the next few hours as the issues get ironed out completely. We would like to extend our deepest apologies for the inconvenience caused by this to your clients. We’re in the process of rewriting some of the code to ensure that the dependability on some of our providers is not so absolute.
As an edit to address follow up questions:
This DNS issue would affect certain mail delivery and processing speeds. But we’re flushing through it as quickly as possible.