Is there ever justification for 'red flagging' emails?
I have to admit, when I get an email with a red flag stating it is 'important', it raises antibodies with me. I strongly suspect the person at the other end has a dysfunctional relationship with email and doesn't fully understand how communication works. Of course, some of this is just workplace norms, and people get subjected to this in their organisation or institution, and the only way to get by is to roll with it.
That said, I'm not letting people off the hook. It might be important to you, but did you even pause for a nano-second to think about whether it is important to the other person? And it's easy to imagine how it's used as a micro-bullying mechanism by asshole bosses. I got an email a few weeks ago related to a conference which had a deadline several weeks in the future and it had an important flag on it. Bizarre. Like I said, antibodies.
It also brings me to another point about email: has anyone ever received an email where someone flagged it as 'low' importance? It's possible to do. One might wonder whether the email needs sending at all but, arguably, it is commendably honest. I might start sending a few and see if anyone notices.