Signing off an email with ‘Best,’. That’s not a thing.
The old ‘yours sincerely / yours faithfully’ protocols were clearly on the move long before business email replaced letters created in the typing pool.
They were replaced by a number of things: regards, best regards, cheers, or just nothing at all. Or a range of accepted alternatives.
But nowhere in the rule book does it say that signing off a missive with “Best,” is acceptable. Best what? Best of luck? Sunday best? Best wishes? All the best? Pint of best? And is it really so hard to write the pertinent phrase? Those seven missing characters saving you important time, are they?
Clearly they are, at least for the very worst type of correspondent. The sort of person who causes the red mist to descend with their ludicrous informality and their downright rudeness.