Similar to other requests about legitimate emails being blocked by MessageLabs, it looks like some of your MX hosts are blocking testing from CheckTLS.com (see below, 5 of 8 are blocked). Either that or you have some failing MX hosts.
I understand why your automated systems may see testing from CheckTLS as a threat.
CheckTLS users, which include some of the largest financial institutions, health systems, insurers, and law offices world wide, do thorough testing of domains, some of which are protected by MessageLabs. These tests probe every MX they can find looking at security. No test ever actually sends an email (we have a strong abuse policy).
CheckTLS has been testing for 9 years, growing 50% per year, and is reaching critical mass in the industry. We do over a million tests a month now.
But from MessageLab’s viewpoint, you see more and more tests, targeting every one of your hosts, that never send an email. I suspect this looks like an attack to you.
Let me assure you, these tests are not an attack. They are not a threat. In fact, they are good for MessageLabs and Symantec. It means more and more people are checking you out.
These are either paying customers verifying that MessageLabs is doing what they say they do, or
outside companies who email MessageLabs paying customers who are verifying that MessageLabs is secure, or
potential MessageLabs customers who are looking at how MessageLabs works.
All this long message is to respectfully request that MessageLabs and Symantec white list CheckTLS.com. In as many places as you can. We do not send email, we do not spam, we are not a hacker site. Our users only have access to the test we publish on our web site.
Please contact me personally if you have any questions or concerns.