Edit: And if I were to theorize that all POP providers acted similarly, I'd be supported by the Mail option referenced in my response to joemike.
I think in this case theorizing would be equivalent to
assume.
It is far more likely each ISP would have unique POP retention policies.
And, of course, the option in reverse wouldn't be of any use, because Apple can't overcome an ISP's retention policy.
Edit: I'm slow on the uptake (for a change
) today. By definition, a theory can't be an assumption. (An assumption is far more likely to get you into trouble than is a theory.)