how does mutt (the code) determine unread email?

[Kurt Hackenberg]

On Fri, Jun 05, 2026 at 10:17 -0500, Ranjan Maitra via Mutt-users wrote:

>I use the Maildir++ format with mutt....

>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 get this information from? Is it from the access time of 
>the file in cur which if later than the creation/edited time for the 
>file is assumed that the file is read? Or something else?

I believe Mutt uses the maildir mechanism: when it displays a message, 
it sets that message's flag "seen", by adding the character 'S' in an 
optional short string near the end of the message filename.

See Wikipedia on maildir:
<https://en.wikipedia.org/wiki/Maildir>