Priorities and Messages
Values are what our lives resolve around. Sometimes, they hide behind reasons, arguments and authorities, but they are always there — and it is us who bring them to the world.
Some things are more important for me than others. I prefer hiking to watching TV. For you, the TV set may be more important.
In contrast, all email messages seem to be equal. Of course, they are not. Any mail from a friend is more important than twenty newsletters. Spam is not as valuable to me as the feedback from you. Any urgent message that requires immediate action is more important than a blast I can read later.
This applies to the messages I receive. But the emails I write differ in importance as well. If I write a friend to ask whether she'd like to join me hiking this is more important than a nice site I forward to myself for later. The chess game I play via email is never as important as an invoice or receipt.
Internet email has a feature that allows sending that importance together with the message. Two header fields can hold priority information. The nonstandard but generally used X-Priority: field and the experimental Importance: header field mentioned in RFC 2421. You should not care about these fields, though.
Communicating the Importance
Most email clients allow you to set the message priority when you compose a message, and you should make use of this feature. Use it to indicate if an email is of extraordinary importance to you, but I think it is even more important (sic!) to indicate when a message is not that important.
The recipient's email client will in some way indicate the importance you assigned to a message. Messages carrying highest importance may be bolded in the Inbox, or marked red while less important messages can be grayed out or moved down the list, for example.
This information can help the recipient use email more efficiently. Of course, the importance attached to a messages does not show the recipient how important a message is for her (and not how important it should be either), but it indicates how important it is for the sender, and that's a lot already.
Communicating the importance of a message is as important with email as it is in face-to-face contact, and it is not much more difficult: assigning high or -- even more important -- low priority when you send a message is all it takes.