[qmailtoaster] 511 sorry, can't find a valid MX for sender domain (#5.1.1 - chkuser) 554 5.0.0 Service unavailable

[Richard Baxant]

I have an external ip address that goes through my router (hardware
firewall) to my webserver box (192.168.x.1) and my qmailtoaster box
(192.168.x.2). I can send and receive email through my qmailtoaster box
without any issues. The problem is when I try to send mail form my webserver
box and I get the error: <<< 511 sorry, can't find a valid MX for sender
domain (#5.1.1 - chkuser)
554 5.0.0 Service unavailable. What am I missing here?