

Depends on the watermark method used. Some people talk about watermarking by subtly adjusting the words used. Like if there’s 5 synonyms and you pick the 1st synonym, next word you pick the 3rd synonym. To check the watermark you have to access to the model and probabilities to see if it matches that. The tricky part about this is that the model can change and so can the probabilities and other things I don’t fully understand.
I’ve been running my own mail for 10+ years. I recommend rspamd for spam filtering. It took the place of SpamAssasin, grey listing, SPF checking, etc. All in one single system.