Previously I knew that this letter has been dated variously from the first to the third century, but just today I read that some scholars recently judge that it is most likely a Christian composition dating to the fourth century.
While the Syriac letter of Mara bar-Serapion frequently comes up when discussing non-Christian references to Jesus, there is precious little recent scholarly interpretation of the letter and its context online. (A little searching does, however, turn up a conference report from 2009, a webpage produced prior to that conference, and a brief exchange on Crosstalk from 2000.) The dating of the letter to 73 AD (or “later than 73 AD”) is widely cited, but most writers online either don’t know why it’s dated then or just choose not to discuss the reasoning.
The reasoning isn’t hard to follow, on the view that the letter is genuine: