Other than his fiancee and his parents, Dietrich Bonhoeffer was not allowed to write anyone.  He was, however, allowed to receive letters.  For some, it did not seem advisable to communicate through letters that might draw the attention of the Gestapo, which controlled the postal service.  Two guard were stationed there especially for Bonhoeffer.

They smuggled letters out of prison to (Eberhard) Bethge and let others go through to particular addresses.

If this had been discovered, they could have expected severe punishment.

(Renate Bethge, Dietrich Bonhoeffer: A Brief Life, 66).