"This is a simple program that displays desktop notifications for unread
mail (or actually any search query) in the notmuch database. The
notification can optionally have a button to run a mail client.

When a message is shown in a notification, it is internally marked as
'recently seen' and not shown again for two days (configurable)."

https://github.com/kspi/notifymuch

There is an AUR package available, called 'notifymuch-git'.