I have several friends that have email addresses which when we test standing next to each other work fine. When I am at home I receive their messages fine on my notebook (outlook, australian domain hosted mail server) but replies from my notebook almost never work (but occasionally do!). Replies from my phone I think work most of the time (using the same australian mail server though, Not a japanese mail address / server).
Someone said something vague about mail from a phone can only be replied to by a phone but how in the hell can that be true from a technical perspective and why in the hell would anyone set that up?
Granted the two people I specifically have problems with are quite old and there is a decent chance of technical competency being the root cause but even by western standards they would be considered pretty tech savvy for older people.