An English version of the Armistice agreement can be accessed via the Wikipedia article "Armistice between Russia and the Central Powers". It provides, among other things, for the exchange of unsealed mail at the front("between sunrise and sunset") and sets up a commission in Petrograd to work out the details.
This provisional measure is very different from a full resumption of postal relations, with mail heading by train to Petrograd from Berlin and vice versa. It basically expresses an intention to give civilians access to the military field post at front-line exchange points.
The collection of Harry von Hofmann sold recently at Heinrich Koehler in Wiesbaden included some examples of such Armistice mail from the German side. Some of it is puzzling and in the absence of the text of the arrangements agreed by the Petrograd commission, it's necessary to guess what is happening, though Alexander Epstein published some information in Yamchik (June 2005).
Here are three cards which belong to this category of Armistice mail: