Did they have sorting offices in neutral countries? Or did they agree to have areas along the front lines where postal vehicles were allowed through?
I was particularly thinking for prisoners of war.
But also, could I have sent a German family member a letter during the period. Would someone have to check what I had written?
It would be great to hear more information, but this old question from /u/usaf2222 has some answers by /u/Georgy_K_Zhukov and /u/kwgoodlet that might be helpful. The question is largely in regards to POWs, but the answers seem to be at least somewhat general to correspondence between the different sides of the war.
To sum up their answers, mail could be passed through neutral countries like Switzerland and Sweden. Charitable organizations like the Red Cross also played a role for POWs, but I can't tell based on the responses whether that was specific to POWs or also available to others.