450 4.7.1
Greylisting is an anti-spam measure employed by some email providers like Gmail.
http://en.wikipedia.org/wiki/Greylisting
The receiving server rejects the message and waits for the sending server to re-send it for a time interval (5 minutes for Gmail). If it receives it for a second time inside this interval, then it whitelists the sending server, which from then onwards can send emails to the receiving server without further rejections.
If the receiving server does not receive the message for a second time inside the predetermined time interval, it rejects the message and the whole procedure repeats itself. It looks like this is the case with your problem, you got these messages stuck in a loop.
Try to find out the greylisting timeframe for Qmail, then adjust your Exchange server's re-send period accordingly (by default it is 10 mins.)
