One of our users started getting this error yesterday as well when sending
out meeting requests:

The following recipient(s) could not be reached:

'Internal User' on 1/7/2009 2:55 PM
This message could not be sent. Try sending the message again
later, or contact your network administrator. Error is
[0x80070057-00000000-00000000].

'External User' on 1/7/2009 2:55 PM
This message could not be sent. Try sending the message again
later, or contact your network administrator. Error is
[0x80070057-00000000-00000000].

If you are getting this error and you are running in Exchange cached mode
close Outlook and rename the OST file in
C:\Users\UserName\AppData\Local\Microsoft\Outlook to NameOfFile.old then
reopen Outlook.

Doing this will recreate the OST file which is a locally cached copy of the
users mailbox, more than likely this will resolve your issues with this...

Steve Di Bias

url:http://ureader.com/msg/10723070.aspx