How do I filter or search email messages in Mutt by a flag?
For example , if I want to list only unseen/unread emails? I know you can use some sort of expressions when doing a search with / but not sure what is supported. Am I able to specify a flag based search for example?
Sounds like you're looking for the "limit" command. E.g. type "l~U" (that starts with an ell for "limit", not a 1 or an I....) to get all unread ("new") messages.
The Mutt Manual is very good. See section 3.1 for details on search expressions like "~U" for "Unread".
Update: I've fixed this to use "U" for Unread, rather than "N" for New.