If you sent him a message of any kind earlier in the season, he could have simply hit the reply button, changed the header and written the new message.
Something like this happened to me in another Strato game last week. But it was a 1st-time manager making the proposal who clearly had no understanding of how the salary cap worked as his trade proposal would have put him $300,000 over the salary cap. Still, I was surprised at how easy it seemed to make a "lopsided" trade work (in this case, an $8 mil pitcher for a $3 mil pitcher was the main swap target, plus a bunch of lopsided hitter trades)-- the guy did have $1.5 mil spare in cash. He was also overexcited about being within 2 games of the lead in his division with 30 to go.
KO, it's collusion. Clever in that it's just a feeler at this stage, but still collusion.
Revealing his name is a tough call. I'd definitely have a private word with the guy first. Perhaps there is an explanation. Ultimately, I'd be more inclined to name names precisely because it is a "well-known" manager. No one likes a cheat of any stripe; however, a cheat who consistently wins and has earned the respect of his peer group is the worst cheat of all.