We need some help with some odd Exchange issues we've been having as of late. Every now and then, out of the blue, Exchange seems to not receive incoming mail.. we start experiencing very long delays receiving messages, some make it, some don't. There don't seem to be bounce-backs or anything of that nature either, and the only clue I have is the following message which was extracted from a sendmail mail log from a server that sends us messages:
Oct 18 16:18:22 web1 postfix/smtp: 1B8EF18182F8: to=<email@example.com>, relay=smtp.dnsexit.com[126.96.36.199]:25, delay=5130, delays=4855/0.01/274/0.13, dsn=4.3.0, status=deferred (host smtp.dnsexit.com[188.8.131.52] said: 451 4.3.0 Please deliver bar.com emails directly to primary MX that is up at this moment (in reply to RCPT TO command))
(I have replaced the domain/ip info in the message above, but everything else is intact and reads exactly in that manner)
I am not an expert on mailers and/or Exchange, so I'm kind of at a loss as to where to go from here, but any help troubleshooting this and finding a resolution would be extremely appreciated.