It's been my experience that these problems most likely occur when:

1) secondary MX hosts are involved

2) security db is unavailable for system maintenance/backup


>>At 02:30 5/24/2005 Tuesday, Moshe Barak wrote:
>>  1 05/22 05/22    Last error: 5.1.1 550 <[log in to unmask]>:
>>                            Recipient address rejected: User unknown in local recipient table