It might be worth ringing your ISP to see if that message comes from some sort of proxy server, router or firewall they are running. If they say it is, it at least eliminates Malware as a worrying proposition. Tell them you've contacted me and I say it's not coming from our server
Problem is the might deny it, even if it is them because :-
1: The person that answers the phone is not technical enough.
2: They are doing it, but it's badly set up and they are trying to avoid blame.
3: They are doing it because they are doing some sort of borderline illegal traffic shaping or something and don't want to admit it.
4: It's a 3rd party doing it not them - got anyone in your area the NSA would be interested in?
To know the meaning of life you need not to be dealt a good hand, but rather to play the worthless hand you have been dealt to the best of your ability.