> 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.
I'm guessing that the way your setup works is that you're running mutt
to read the mail locally, while at the same time you have an IMAP
server set up that can also read the email. When you access the email
directly (i.e. not with IMAP), you aren't asking the IMAP server for a
UID, so it isn't generating unique identifiers and saving them in the
emails. But when you use an IMAP client, to check the email---even if
you don't actually read the email---then the IMAP server adds the
X-UID tag to it.
This is just a wild guess, though. It may be something entirely
different.