Husband Blindsided by Separation ...

Mia Thorntons estranged husband, Gordon, did not see their separation coming -- cause the guy thought everything was fine and dandy until he got served ... TMZ has learned.

Sources close to the hubby of the RHOP star tell us Gordon was handed court docs on Mias behalf back in mid-August ... which signaled her desire for legal separation. Were told the move completely blindsided him -- and that he had no clue it was coming.

Our sources tell us that Gordon is also peeved right now over Mias public handling of the situation -- namely because were told he feels like shes been BSing everyone.

Fact is ... until Friday -- when MT announced they were separating -- shed made it seem like they were A-OK on social media, continuing to post photos of the two of them together and even shouting him out a handful of times ... calling him a positive influence on their kids.

Our sources say Gordon is irritated by this ... and not just that, but were told he hasnt even signed the papers yet. Her going public can be viewed as pressure on him to do so.

Mia broke the news to People, saying ... "Like all marriages, ours has had its ups and downs. At this time, Gordon and I are separated, but my family is my number one priority, and I am committed to doing whatever it takes to make sure we are okay. We ask that you respect our privacy and give us the time and space to figure things out."

Of course, the backdrop of all this is the fact that "Real Housewives of Potomac" has a new season on the horizon ... its set to premiere in November, and this is sure to be covered.

Stay tuned ...