OK, here's the setup: we have two Xserves, with XSan set up as a clustered mail server. The OS is 10.6 Server, fully up to date. We're using this as a replacement for our dying mail server. Now, before I get too far into this, we do have a handful of mail users on a 10.5 Server... mostly our power users whose large mailboxes were an issue on the aging mail server. So far I have two accounts set up on the Open Directory that the mail servers are using, me and my boss. We're receiving mail just fine (we've set our enterprise LDAP to deliver mail via SMTP to both the old and new servers for the two of us for now), and we can email each other. But, if I attempt to use the new Mac mail cluster to send mail to anyone other than my boss, I get: 550 5.1.1 <email@example.com>: Recipient address rejected: User unknown in local recipient table And yes, that's true, it's not defined in the local table, because it's an account we haven't migrated over yet. The expected and desired behavior is that it will take any email that it can't deliver locally and send it to the server defined in "Relay outgoing mail through host:". I say expected, because this is exactly how our 10.5 server works. We can point out clients to it for outgoing mail, and what it can't deliver locally, it sends out to the mail relay, which gets it to where it's needed. And yes, I've compared the mail configuration screens of the 10.5 and the 10.6 box, and in every way (except where 10.6 brought a new option), they're identical (other than the host name). Does anyone have an idea? This isn't just a transition issue; our domain is shared amongst dozens off offices with thousands of users, and only 200 of them will be on this server.