how does mutt (the code) determine unread email?

Hi,

I use the Maildir++ format with mutt. It all works very well: I have been =
using xbuffy (also gbuffy) to monitor my mailboxes for at least two decade=
s but these programs currently only either new mail or all mail. I would l=
ike to change this to include unread email.=20

Now it is my understanding that mutt moves accessed email from new to cur =
when a folder is accessed. However, the coloring appears to indicate that =
the mail is not read (unless we go to that message/file). Where does it ge=
t this information from? Is it from the access time of the file in cur whi=
ch if later than the creation/edited time for the file is assumed that the=
 file is read? Or something else?

If I can figure this out, perhaps I can modify gbuffy or xbuffy to include=
 unread email in the monitoring.

Many thanks for your help, and best wishes,
Ranjan