Activity Monitor showing long duration for WAITFOR RECEIVE
This is a red herring. WAITFOR (RECEIVE...) is by definition supposed to ... wait for messages! Therefore 281 seconds elapsed simply means that for 281 seconds there was no message to receive. In this case the application is the Database Mail external sending process that is sitting idle waiting for messages to be enqueued (ie. waits for sp_send_db_mail to be called). There is nothing to 'resolve' here.