CompanyA running Exchange Server 2000 and attempting to send mail but getting delivery failures to CompanyB which has two MX records:
mail1.companyb.com, preference 5
mail2.companyb.com, preference 10
If I telnet to port 25 of mail1.companyb.com, it responds with:
550 Requested action not taken: mailbox unavailable
Then it drops the connection.
CompanyA's Exchange server never attempts to go to mail2.companyb.com to deliver mail. Instead, it just issues all users at CompanyA a delay, and then eventually a failure.
I have other clients running Exchange 2003 server, and they can deliver mail to CompanyB. Is there a setting on the Exchange 2000 server?