Sudden Deferred: Connection Refused by [destination host]

Suddenly three days ago, one user at one domain cannot send mail into one email address at one domain on my server.

User1@othercompany.com cannot send mail to UserB@oneofmydomains.com

First, this message, then it eventually fails.

This error was caused by an e-mail sent from somebody@micemetery.com

**********************************************
** THIS IS A WARNING MESSAGE ONLY **
** YOU DO NOT NEED TO RESEND YOUR MESSAGE **
**********************************************

The original message was received at Fri, 5 Oct 2007 13:09:01 -0400
from ns-omr2.mgt.netsol.com [10.49.6.65]

----- Transcript of session follows -----
<Danae@staffcos.com>... Deferred: Connection refused by staffcos.com.
Warning: message still undelivered after 4 hours
Will keep trying until message is 5 hours old


Any help would be appreciated, somebody@micemetery.com *can* send messages to otherpeople@staffcos.com


Thanks.
Jason

 

How so? it only happens when certain people from the that other domain send mail to one person at the domain on m y server.

Can greylisting work for only one address?