I recently got a new Fiber line in the office to replace our current provider. Upon testing, I have discovered a small problem with the mail system.
The configuration is as such: We have an Exchange 2010 server that sends out all our mail. For incoming mail ONLY, we use Microsoft Forefront (outgoing mail does not pass through forefront). The reverse DNS lookup on our external mail IP goes to mail.ourdomain.com which then points to Forefront. This has always been the case (the mechanics of the environment haven't changed, just the external IP address)
The problem is that now, we have people who cannot send email to recipients at ONE domain (they too are using Forefront for their MX record). They receive a Relay Access Denied error from our Exchange server. All emails to other domains are sending problem free. This issue did not exist prior to changing the ISP.
I have tried changing the reverse DNS to point to our external mail IP (no success) and I have telnetted into both my mail server and the recipient mail server and neither shows a Relay Access Denied when I type in the rcpt to: command.
Does anyone have any idea?