I never fill that field and always tell everyone to just ignore it
Turns out it can! When you don't edit it, there is a red star in front of the label, leading me to think it can't be left empty. But, now I'm left with another problem: I can also leave the 'From' field in the worflow e-mail task empty, so if there is some task where someone didn't enter a 'from' e-mail address: I have no escalation and no idea what will happen.
Fixed that. Thanks!
I would say that it will use the current users email