Ok I have one that I’ll have to adapt slightly in case it’s recognised, but:
DH works in a big office in town. They’re shared offices, so while he works alongside people on his floor, they don’t work for the same companies.
A married woman on his floor is known for sleeping with other married coworkers, which is made even more awkward by the fact that her husband also works in the building, albeit on a different floor.
Recently, she started getting internal mail from a disgruntled wife, calling her lots of names and accusing her of being a homewrecker and a whore. It was distressing for her, and an investigation was set up to try and see who was the source of it, but it could have been anyone from any of the businesses using the building and the internal mail. It was all kept quiet while they investigated, and she couldn’t even go to her husband for support as then she’d be admitting to the affairs and not knowing which wife she’d pissed off.
Well, they managed to use security cameras to eventually track down the letter sender. It was her own husband!