Email Relay from outside server

I have an Exchange server running emails for a sister company that I created and open relay for in the past and has worked perfectly fine. Until I added a new ISP at the site of the exchange server. All of our host records have been updated. My problem now is that I can receive inter-company emails relayed from the outside server, but I get a failure on sending outside emails form that server.

all other emails handled locally at the exchange server site work perfectly fine.

Any ideas why internal emails will relay but not external just from changing the ISP? Our old ISP is still connected for failover.
Dennis JansonIT ManagerAsked:
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Will SzymkowskiSenior Solution ArchitectCommented:
For external mail you need to check the Send Connectors. Due to the new ISP the only thing that would have changed is your internet facing IP address. Are you sure your MX records are correct for external mail? Is there anything else in your environment which is dependent on your old ISP IP that they gave you? If you failover to the old ISP connection everything goes back to normal?

Dennis JansonIT ManagerAuthor Commented:
My MX records are correct. Emails for my other 2 domains are working perfectly. What could cause emails from this relay to work for internal domains but fail when sending to external domains?
Dennis JansonIT ManagerAuthor Commented:
Also, I shouldn't say that that the ISP has fail-over in DNS because GODADDY cannot handle that. But it is setup on my Sonicwall Firewall.
Dennis JansonIT ManagerAuthor Commented:
I found the issue. It was a firewall rule on my Sonicwall. Around the same time that our new ISP was added support had changed some of the firewall rules to adjust for anti-spam filtering and it was blocking the SMTP port from the relay server.

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Dennis JansonIT ManagerAuthor Commented:
Issue was due to my own changes on our firewall. It happens.
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