When you try to send a meeting request from Outlook you get the following error :
Your message did not reach some or all of the intended recipients.
Subject: Test Message
Sent: 29/03/2012 13:51
The following recipient(s) cannot be reached:
Smith John (Test) on 29/03/2012 13:51
This message could not be sent. Try sending the message again later, or contact your network administrator. Error is [0x80070057-00000000-00000000].
To fix this you need to ensure that the sender of the request has picked the address from the global address book or if that does not resolve the issue try a /cleanprofile or /cleanfreebusy and this should resolve the issue. I have seen this error on Outlook 2003 and 2007 so it may be on other version also.