You will never find a confirmed answer for this question for a number of reasons.
- Microsoft are a law unto themselves.
- There is no way for you to confirm why. The actual reason for filtering is never released and therefore testing is pretty much useless. Microsoft just give a score and no individual reasons (like spamassassin)
However, it is likely that someone has previously flagged this email address as spam, probably not releasing that they actually signed up for it (and I hope they did). If the content is identical and the only thing that changes is the address, I can think of no other reason.
If it was a content issue, then in theory all emails from any address would get flagged.
If it was an IP reputation, same as above.
This leaves sender address.