X-UID in mutt

["Chen Dennis" <dennis11111@xxxxxxxxxxx>]

Dear all,

I've been looking into X-UID information in mutt for a while. 
It is told by google that X-UID is added to email messages 
AFTER they arrive in your mailbox. It is true for the history 
emails but not for some new emails (arrived less than 1 day).

I'm wondering how long the X-UID will be added to email
messages after they arrived. I've tested sending email from
different accounts to see how it changed, sometimes X-UID
is appended before I open the new email, and sometimes
X-UID is not added even after one day. 

My mutt's version is 1.4.2.1i, and linux 2.6.17 platform.

Any input is greatly appreciated, and thanks for your time.

Thank you,
Dennis