Exchange 2013 - unable to recieve external emails

Hi Experts, have an issue where we are unable to recieve any external email. We had an existing email server and we added a new Exchange server. All URLs match for both servers and so do the certs. Today we made a change to firewall and externals DNS since IP address of new Exchange server has changed and now we are not receiving any external emails

Internal emails and sending emails to external domains is working fine

TIA
abhijitm00Asked:
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Jason CrawfordConnect With a Mentor Transport NinjaCommented:
Yes you only need public DNS record for Autodiscover.  The A record you currently have in place should be fine so remove the SRV.
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Jason CrawfordTransport NinjaCommented:
Where is the current MX record pointing?
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abhijitm00Author Commented:
MX record points to Comodo Antispam gateway so there is no change there
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Jason CrawfordTransport NinjaCommented:
Have you tried running a message trace on the Comodo?  It would be helpful to know if it's ever received by an Exchange server or if it dies at the gateway.

What server is the Comodo handing off to?
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abhijitm00Author Commented:
I am getting: Relay : exchange.domain.org:25
 Error code : 500
 Error message : SMTP host unreachable
 No entries on your request
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Jason CrawfordTransport NinjaCommented:
Ok did you add a firewall ACL and NAT for the Comodo IP on port 25?
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abhijitm00Author Commented:
No there is no ACL for Comodo on the firewall. It is open to all IPs
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abhijitm00Author Commented:
Just want to recap - Existing server name - mail.domain.org. All URLs point to mail.domain.org on both Exchange servers. New server name is exchange.domain.org. But cert has mail.domain.org as primary and exchange.domain.org and autodiscover as SANs.

All external DNS for autodiscover, mail.domain.org (via CNAME) point to exchange.domain.org which has a new external IP. NAT to old server mail.domain.org is stopped. At this time Activesync, Autodiscover and Outlook Anywhere stop working. Sending emails externally and internally work so does recieving emails internally, what does not work is emails sent from outside.

Would this have anything to do with FQDN name on recieve connectors or SSL certs?
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abhijitm00Author Commented:
Also wanted to add to comment above that Comodo Antispam can't seem to connect to exchange.domain.org on port 25 although all firewall rules seem fine
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Jason CrawfordConnect With a Mentor Transport NinjaCommented:
In that case you should be able to test access through the firewall by going to canyouseeme.org while logged on to the server receiving email from the Comodo and specify port 25.  Either that or you can try sending an email with PowerShell while off the network:

Send-MailMessage -From sender@domain.com -To recipient@domain.com -SmtpServer exchange.domain.com -Subject 'Testing' -Body 'This is a test'

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If your firewall truly accepts all connections on port 25 that should at least make it to a Receive Connector.  While you're at it you should enable verbose logging on all Receive Connectors:

https://msdn.microsoft.com/en-us/library/bb124531(v=exchg.160).aspx
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abhijitm00Author Commented:
Ok made some firewall changes and email is indeed flowing to new Exchange server.

Autodiscover and Outlook Anywhere is connecting but only after creating a new profile by manually adding server name. I have changed all URLs to point to exchange.domain.org and removed mail.domain.org from external DNS.
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MB ShaikhCommented:
Hi,

Once if you receive you external email at your Comodo Antispam gateway, then check your newly installed exchange server is accepting connections on port 25 from  Comodo Antispam gateway  vice versa  probably fire wall may be enabled on newly installed exchange server and in Comodo Antispam gateway you allow proper routing etc.

regards,
MB Shaikh.
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abhijitm00Author Commented:
Android and iPhones are not able to configure using autodiscover. If I manually setup profile using server name I am able to connect. Outlook anywhere is working correctly. How can I fix autodiscover for phones? Currently i have autodiscover A record pointing to external IP of exchange.domain.org. There is also a _srv autodiscover record pointing to website, should i change this to exchange.domain.org?
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Jason CrawfordTransport NinjaCommented:
Glad I could help.  Take care :)
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