Send notification to contact or groups when commenting a file

Hi there,
this is my first post on this forum.

My goal is to send an email notification when I comment on a file.
Nextcloud users come from an LDAP AD.

When I search for a contact by his name or email nothing is found.
I added a contact manually from Contact app but it is not found neither.
When I type a comment on a file @username > Unknown user error

Can you give me any tip ?