Complex sentence: The Mongol Empire had an ingenious and efficient mail system for the time, often referred to by scholars as the Yam, which had lavishly-furnished and well-guarded relay posts known as "örtöö" setup all over the Empire.
Split into simpler sentences: The Mongol Empire had an ingenious and efficient mail system for the time, often referred to by scholars as the Yam. It had lavishly furnished and well guarded relay posts known as "örtöö" set up throghout the Empire.