How send email with "execute as user"

I need that all my system users can send email from the DB. not to assign everyone the role in the msdb database I created one login/user "send_mail", and grant permission on msdb db, specifically DatabaseMailUserRole. I need that all my users send email execute a store procedure that within execute the statement execute as user='send_mail" before of msdb..sp_send_db_mail. The user send_mail have db_owner, and the DatabaseMailUserRole, but can't send email. The EXECUTE permission was denied on the object 'sp_send_dbmail', database 'msdb', schema 'dbo'. Anyone can help me please?