How to Recover a Message from Spam in Facebook Messages

To make nouns from adjectives, speakers of an older English added 'es': green became green-s, for instance, clean clean-es and glad, I presume, glad-es.
The adjectives ending with an 'n' abounded. With time, something like "glad-es" began to sound odd and wrong. The suffix turned from 'es' into 'nes' and then 'ness'; glad-ness everywhere!
All right, let's tackle, in Facebook, a message's spam-less-es:
›› Missing an email, message or newsletter perhaps? Maybe it was caught by Facebook Messages's spam filter — and by mistake. Here's how to recover that conversation (and help teach the filter).