The Big Lift

Off-duty American airmen of the 19th Troop Carrier Squadron in Hawaii are ordered to report to their squadron in July 1948. What is briefed as a temporary "training assignment" in the United States becomes a flight halfway around the world to Germany for the C-54 Skymasters of the 19th, where the Soviets have blockaded Berlin in an attempt to force out the Allies by starving the city. Tech Sgt. Danny MacCullough (Montgomery Clift), flight engineer of a C-54 nicknamed The White Hibiscus, is immediately ordered to fly with his crew from Frankfurt into Tempelhof Airport to deliver a load of coal. His friend Master Sgt. Hank Kowalski (Paul Douglas), a ground-controlled approach (GCA) operator, hitches a ride with them to his new station. Hank, a POW during World War II, resents the German people and goes out of his way to be rude and overbearing to them. Danny on the other hand is frustrated at being restricted to the airport because of the necessity of quickly offloading and returning to Frankfurt.
Months later the crew of "Big Easy 37" (a call sign, airlift shorthand for an eastbound C-54) renames their airplane Der Schwarze (The Black) Hibiscus because of the grimy soot that has accumulated in it from hauling coal. They become temporary celebrities on a mission when they are the 100,000th flight of "Operation Vittles" into Berlin. Danny is immediately enamored of Frederica Burkhardt (Cornell Borchers), an attractive German war widow chosen to thank him on behalf of the women of Berlin. When a news correspondent covering the ceremony recruits Danny for a public relations stunt, Danny jumps at the opportunity as a means of getting a pass in Berlin and seeing Frederica again. During a tour of the city, Danny's uniform is accidentally covered with poster paste, forcing him to wear civilian working clothes until it is cleaned. They meet Hank and his "Schatzi", the friendly and intelligent Gerda, at a night club, where Hank is rude to Frederica and treats Gerda as an inferior. Hank chances to see the former prison guard who tortured him as a POW, and beats him nearly to death. Danny is able to stop Hank only by knocking him down. Mistaken for a German attacking Hank, he is chased into the Soviet occupation zone by military police.
Danny and Frederica barely escape back into the American zone, where Hank is waiting for them at Frederica's apartment and has unexpectedly befriended her neighbor and Danny's friend, Herr Stieber (O.E. Hasse), a self-professed Soviet spy. Danny falls in love with Frederica, despite learning from Hank that she lied to him about the backgrounds of her dead husband and father. When Danny receives notice that he is due to rotate back to the United States soon, he arranges to marry Frederica. However, Herr Stieber suspects duplicity in Frederica and intercepts a letter she has written to her German lover living in the United States, revealing that she intends to divorce Danny back in the U.S. as soon as she legally canâand see her lover behind his back until that happens.
In the meantime, Hank, in trying to teach Gerda the meaning of democracy (and deeply ashamed of the beating he inflicted on the former guard), comes to see that he has been hypocritical in his own actions toward Germans. He begins treating Gerda as an equal and with affection as they meet Frederica to be witnesses to the wedding. When Danny arrives, he tells Fredrica she will have to wait a long time, if ever, to get to America. Herr Stieber has given Danny the letter she wrote. Gerda says she prefers to stay in Germany and do her small part in helping rebuild the country, and Hank reveals to Danny that he is not going home but has switched his temporary assignment in Berlin to permanent duty. Danny's flight out departs amidst rumors that the Russians will soon end the blockade.

Question: Who gives Danny her letter?
Answer: Herr stieber