In every mail program you will find an option to set the priority of your emails. On a mac it looks like this:
But what is it good for? If you think it is as a hint for the reader to explain how important this mail is, you are wrong .
This is a "pro active spam filter". In the beginning of email, software developers tried to find a way to distinguish emails by self-important persons from emails that really needed their attention. Since software developers are very expensive, it was not possible to let them answer every mail of those egomaniac persons. Mails like:
Attention! Showstopper! The button is in dark green, but it should be in a lighter green! YOU HAVE TO FIX THIS ASAP!
A normal spam filter was not able to detect those persons. It needed some help to do that. But how? The idea was very simple, but worked out great: a egomaniac will always try to get his mail answered as soon as possible. And he will try everything to cut the line. That's the reason why that "priority" field was invented. The spam filter only has to check the ratio of "high priority" mails to normal mails from every person. If the ratio is higher than a certain amount, the person is considered to be an egomaniac and his mails will be sent to the bottom of the reading list.
I can't live without this. But don't tell any manager/user/customer. It's a secret that saves my ass regularly .
This article is a translated post I wrote a few years ago.