Igor Ryss provides the text and translation as follows:
Pis'ma tol'ko zakaznym prisylat'! - Send only registered letters!
This is important. It shows the sender thought he could receive letters as well as send them
The cover is franked at 1 rubel, so one might guess at 50 kop postage + 50 kop Registration.
We also seem able to add Dobruchinskoe to the list of post offices under North Western Army control, not only because of use of the stamps but because of the backward journey to Tallinn which would not have been possible if this was an item of RSFSR mail.