Prior to the outbreak of WWII, German zeppelins circled the globe delivering passengers and mail.  As sending mail via zeppelin was still a novelty, many flights used distinct postal markings which allow modern collectors to specifically identify the flight and route via which a particular piece of mail traveled.

Zeppelin mail is its own specialized collecting area, with excellent references detailing every flight and stop of each airship, German or otherwise.  The primary references for this area are Michel’s Zeppelin- und Flugpost-Spezial-Katalog 2017/2018, and Sieger’s Zeppelinpost-Katalog.