I think alot of this bullshit continues because there's no record of it. Postie's aren't given a company email for self serving reasons I believe. I don't know if you can message in the app but as access disappears when you leave, it's fcuk all use.
By the time I got really sick of the game, I'd email my manager recording what was said when and by whom. Nice and polite. But factually accurate. Sometimes when the bulling was completely astonishing or I wasn't ready to use it, I'd make my own private notes to use at a later date.
All this bullying is based on a regime of plausible deniability, namely if it's not recorded, it never happened.
I recall one postie a few years ago now, 2017 ish, who was getting the same old same old and one day he turned round to the lino and said, "I've been recording everything you've been saying to me for months you know". They left him totally alone after that.
I know exactly how the original poster feels about the issue. I hope you find a new strategy to get through the day.