I am reading about the dead letter office (a warehouse in Belfast) and undercover mail. Did you know that during the second world war post to enemy countries was sent via Thomas Cook in Lisbon?
"Letters and envelopes must omit the sender's address. They must only refer to matters of personal interest… (a) no reference may be made to any town (other than Lisbon), village, locality, ship or journey… No indication may be given that the writer is not in Portugal… (b) mention of a letter … received from or written to enemy occupied territory is not permitted… " Dr. Josef Wallach.
The same process for undercover letters was used thirty years later by Israel and the Arab states following the 1967 war. I am pretty excited, will have to see if I can find out more in Lisbon at the end of the month.