smtp test - how/what does it actually do?

we ahve a client configured thus

10.108.249.162? colt # ssh ntp MAINTNC:linux smtp

This smtp check has been green for ever - literally. It has suddenly alerted overnight and is remaining so


Service smtp on colt is not OK : Service listening but unavailable (connect timeout)
Seconds: 0.001814000


I can telnet from the xymon server on port 25 fine.
postfix services on colt are working seemingly correctly? (eg i can send a mail successfully from a client that uses colt as a smtp server)
So Im now trying to find out what it is that this smtp check atually does in order to find out why it now thinks it cant do it.

The error suggests indeed its a timeout issue - but the telnet test connects immeditaely (no obvious lag anyway)
thoughts/
ian