How was mail physically delivered between the opposing sides in World War II ?
After the German conquest of Europe, Britain was isolated against the Axis powers and yet there were certainly mail deliveries during this time. How was this achieved? Through neutral Ireland or Spain or Portugal perhaps?
Officially? No. There was no mail between opposing sides except perhaps diplomatic telegrams. At the start of the war the Germans immediately began attacking British shipping in the hopes of paralyzing and disrupting her supply lines. I cannot conceive that they would have made an exception for the mail.
Unofficially, people sent mail through friends in neutral countries who would re-post it to the final destination. It was common practice to double-wrap such parcels so the intermediary would not have to completely repackage them.
Germany had centralized censorship offices for handling mail to enemy states. That suggests that (1) the load was small enough to be manageable, (2) if wasn’t banned, and (3) the post office had some way of arranging delivery through a neutral state.
Back before email, a lot of european mail went through Switzerland even through CH was nowhere near the expected path and if you were mailing from Europe to the US and viceversa one of the waypoint stamps on the envelope would be from Greenland. Like air paths, mail traffic is one of those things which doesn’t necessarily take the route someone would guess. Nowadays a lot of those stamps have disappeared, not because the letters take simpler paths, but because of Schengen reducing the amount of customs points they go through.