Depending on the email software being used, an emailer may be able to indicate to the reader that a message is of high priority by marking it with a red exclamation point or other urgent-looking symbol. This feature tends to be abused by senders whose idea of a high priority doesn’t necessarily correspond to their readers’. Unless you are absolutely certain that the situation is urgent, don’t mark an email as high-priority — particularly if it is going to someone you work for.
Even when it is urgent, you have alternatives to the red exclamation point. For example, you can send a regular (i.e., not high-priority) email and then follow up with a call.