I like to include, as a good practice, X-Priority in my headers. While all email messages seem to be equal they are not. For me an mail from a friend or from my website's contact form is more important than for example a newsletter, so depending on the project im working on i change the X-priority value, which btw ranges from 1 to 5, with 1 being high priority, 3 normal, and 5 low.

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