While Martin's character arc in Address Unknown demonstrates that he is unfaithful and self-serving, Max proves to be resilient and a seeker of justice.
Max and Martin are business partners who have constructed a successful company together. When the men first part ways, their letters are warm and affectionate, sending love from America to Germany and back again. However, once Hitler rises to power, Martin's loyalties shift significantly.
The mood becomes increasingly dark as Martin begins to praise Hitler. He admits to joining the regime as an "official" and feels that Hitler will sweep Germans into the destiny they deserve. In his letter dated July 9, 1933, it becomes evident that Martin has incorporated the beliefs of the Nazi party as his own values; he writes that the Jews didn't become the "universal scapegoat" without reason and that he has only loved his friend Max "in spite of" the fact that Max is Jewish. By contrast, he praises Hitler for being "gentle" and asserts that Germany needs to "cleanse" itself of "its baser elements"—which are people like Max.
At first, Max believes in the friendship he and Martin have shared. He writes to Martin praising him for his "warm heart" and is certain that his friend will tolerate no "viciousness." He believes that Martin is honest and reminds him that he is a forever "faithful" friend. When Martin completely betrays him, refusing to help Max's sister Griselle when she comes to him for help, Max retaliates. He begins sending letters to Martin's home address again, which Martin had previously begged him not to do; he feared that Nazi soldiers who read personal mail correspondence would realize that Martin associated with a Jewish man, which might implicate him in forbidden activities. Max's letters also include cryptic messages, full of numbers, which are presumably well-crafted to heighten the suspicion toward Martin by anyone who intercepted those letters. Additionally, he includes a reference to their "grandma's" birthday, which would indicate that Martin and Max might share Jewish ancestry. This demonstrates Max's cunning abilities to predict both the actions of the Nazi regime and to exact revenge on his former friend for allowing Griselle to die without attempting to help her. Martin, by contrast, transforms again, begging Max not to spare him and his family. In the end, it is Max who holds the power, and he uses it for vengeance.
When considering the effectiveness of the final letters, you might want to consider how they are used to accomplish Max's purpose. Why is it effective that none of those letters receive a reply? What can we assume has happened to Martin, and does that provide effective closure to this conflict? Why is a closing line such as "the God of Moses be at your right hand" a particularly effective line to include, both religiously and purposefully?