Hi Dusan,
Did you follow the instruction I posted above and raised this issue in the other forum:
https://www.symantec.com/connect/security/forums/messaging-gateway
This Forum is for .cloud customers thats use the messagelabs infrastructure. I have checked both our internal Blacklists and the reputation across our Infrastructure and I can confirm that we are not blocking your connections.
If you have any logs stating otherwise please do let me know, however I believe your issue resides in an area of the business that we at .cloud do not have access too.
Please do let me know if you have other information to assist me or if the problem gets resolved in the other forun please mark this thread as resolved.
Sorry if you think I am being unhelpful but from the information I have before me I cannot see why we would block your connections.
Kind regards
Kevin Brosnan
Tier 2 Senior Technical Support Engineer.