The Deutschland made two voyages to the U.S. In the summer of 1916, she delivered a cargo of chemical dyes, medical drugs, gems, and mail to Baltimore, Maryland, then returned to Bremerhaven with a cargo of nickel, tin, and crude rubber. In the fall of 1916, she delivered a cargo of gems, securities, and medicinal products to New London, Connecticut, and returned with a cargo of silver bullion. Before the Deutschland could embark on a third voyage, relations between Germany and the U.S. deteriorated significantly, so she was converted to an armed U-boat, the U-155.
The Bremen departed Bremerhaven in September 1916 for Norfolk, Virginia. She reportedly carried funds which were to be used to pay an American firm to build more cargo submarines for Germany. She never arrived in Virginia, and her fate remains unknown.
For the eventually-cancelled third voyage of the Deutschland, the German postal authorities offered to carry regular mail (as opposed to high-value mail requiring insurance) originating from within any of the Central Powers. These Tauchbootbrief letters contained two parts, the outer envelope and the inner envelope. To send mail by this manner, the sender had to attach the proper postage for normal foreign post and have it cancelled at the post office of origin, then place the entire letter, unsealed, in an outer envelope bearing only the words “Tauchbootbrief nach Bremen” (submarine letter to Bremen). The outer envelope was then franked with a 2 Mark stamp to cover the cost of the service.
The inner envelopes were cancelled with a special Bremen cancel. Upon the cancellation of the submarine mail service, however, the Tauchbootbrief received a refund cachet and were returned to sender.
Tauchbootbrief are known from Germany, Austria, Hungary, and Bavaria. None are known to exist from Turkey.