I had been testing out a less iron fisted firewall policy ...
For the week or so it lasted.
It seems that the scum which exists along side us all in the internet is simply without any redeeming virtue at all and therefore cannot be given quarter from any manner of cutthroat security policies which may be implemented for the safety of the domain.
Therefore, I have resumed the original scorch the earth policy on the mail server that is still in place on the web server.