When it hit that limit... everything stopped until we could free up some space
I know that feeling, too. But with Exchange it was a case of "everything stopped and God couldn't make it work again".
There may have been ways to coax it back to life, but I didn't take the chance. When Exchange 2003 SP2 came out, I'd always set the limit to 72GB in the registry, keeping those 3GB in hand for the inevitable emergency when it would hit the stops.
Telling the boss "all of the company email will stop in 4 months if nobody clears out" had the desired effect (unusually). That company was still using Exchange 2003 in 2012, with the same 72GB mailstore limit...