the problem is with the target server, not yours - it's a
generally known bug of a misconfigured MS Exchange server. To be
able to send mail there you have to add a line into your
esmtproutes config file with (for example):
Try to google for it and you'll find various links to posts about
this Exchange behaviour. I have found many of them at Exim mail
I have this entry and restarted courier but still get the same error.