How can I mark an email as important?

Question

I am using PHP with the Zend-Framework to create emails. How can I mark the email as important? Outlook will show a red "!" if it is marked right.

Answer 1

I am not familiar with Zend, but you are looking for these headers:

X-Priority: 1 
X-MSMail-Priority: High
Importance: High

According to this page Outlook sets all these headers when you send an Important email, but I suspect that the first one would suffice.