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I first noticed this while working at HostGator. Routinely customers would open tickets about emails disappearing into thin air while targeting Outlook/Hotmail. In almost all cases I was able to replicate it from their domains.
Naturally, I started to try to narrow down the cause to find out if it could be IP reputation, domain reputation, or content. If I could send an email from their domain that was a personal email from me to myself, I'd pretty well rule out content. It would persist, so ruled out. I'd bring in one of my own test domains and add it to their VPS (where the customer had a VPS and therefore a dedicated IP for mailing) and send myself an email from that domain, I'd get it in my inbox. So that ruled out IP reputation. In my tests, domain reputation seemed to be my conclusion.
That said, it wasn't a large enough sample size to say it's the only reason. A customer of mine at MXroute once noticed the same behavior and they cleared it up by changing their Content-Type header. I can't recall which header they'd set and what they changed it to, but the client was someone I'd trust.
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