i've got my tags synced between thunderbird and zimbra, so that made me think the tags were actually stored in the headers of the email (using imap keywords?)

however, when i view the full headers of the message, i don't see my tags.

i use mutt when i work, but i don't have a way of customizing mutt based on tags, because i can't find the tags in the email headers.

does someone know where they are, so i could build rules around them in mutt?