Grey listing is an anti spam measure implemented by the "other" exchange server in the recipeint domain
Read the below mentioned article as it clealry explains what is "Grey Listing"
There is a known issue wherein sometimes these messages are ust "lost"
There is a hotfix with a reg mod for "Glitch Retry Interval" that can be set.
Once you go through this article you will understand





by: anupnellipPosted on 2009-05-15 at 01:46:06ID: 24393296
Doest your mail server retry mails after some time . the other server si jusr graylistin your mails
Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again and the email is accepted. If the mail is from a spammer it will probably not be retried since a spammer goes through thousands of email addresses and cannot afford the time delay to retry
if you server retrys to send the mail again it should work .