When making telnet to port 25 to the email prevent server from upstream MTA getting the below error:
[Telnet] Fatal: Forwarding agent unavailable. Closing Connection
Error messages found in SMTP logs of detection server:
16/Apr/19:08:38:17:321+0300 [SEVERE] (SMTP_CONNECTION.5203) Forward connection error (tid=3f cid=b15a4fed-e11e-4e52-a6c9-8aae25c90103 mta=cluster4.eu.messagelabs.com reason=Connection refused: no further information)
16/Apr/19:08:38:17:321+0300 [SEVERE] (SMTP_CONNECTION.5210) All forward hosts unavailable (tid=3f cid=<> reason=No available forward hosts)
Try clearing DNS cache - often it resolves the issue.
If this doesn't help - contact support to check are there any issues on Email Security.cloud (aka Messagelabs) side as the problem might be related to communication on their side.