Hah, 5 minutos depois de postar a pergunta, eu mesmo encontrei a resposta. Já aconteceu isso com você? : -)
Essencialmente:
It took a few calls with AboveNet (now part of Zayo), but we were finally able to determine that one of their customers is an anti-malware firm based in the UK, providing services to two of our common customers. They were scanning all incoming emails and probing any hyperlinks to identify potential hazards and/or vulnerabilities in the destinations.