One question that’s always been burning in my mind is : MAIL. Obviously letters could still be sent between Axis and Allied Countries. So where there planes or ships that were protected / off limits from attack that went between say France/Germany to and from England for example? When someone on the Axis side wanted to ask for say a prisoner swap, peace feelers or safe passage for a civilian passenger ship etc, what was utilised as the conduit for this?
Mail was typically sent through a neutral country say for instance Switzerland to get to or from axis or allied countries however almost all mail was opened and inspected more than once for hidden messages or codes.
It would of course take considerably longer than mail normally would as it often times could be 3 months before you would see mail delivered from elsewhere and likely been opened and sometimes letters would be delivered with sections blacked out by the censors who read the mail and sometimes the mail would be destroyed for reasons.
When it came to POW mail and care packages it was often handled by the Red Cross who were supposed to be neutral in the conflict which is why they were allowed to cross enemy lines but again they were subject to searches as well.