How to grant sendmail permission to sql server user?

I have a database user which is the owner of the database. The application requirement is to send mail using database mail of sql server .

Is there any way that I can add grant only send mail permission to that user?

I have a user named testuser having server roles public and is db_owner for 1 database. Please tell me the way that I don`t need to give sysadmin serverroles to that user.