<div>
so if a user on the comcast.net platform, emails<br />
<br />
1. old server<br />
2. new server<br />
<br />
it fails ?
</div>


<div>
Please post the exact errors you are receiving.
</div>


<div>
Sounds like a Comcast DNS problem.<br />
I've had users in the past have emails get lost because of ghost DNS &nbsp;issues with the ISP(Network Solutions).<br />
Call up Comcast and see if they can duplicate the issue.
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<div>
within COMCAST what MX records do they have for your mail servers ?
</div>


<div>
Thanks for the replies.<br />
<br />
Here is one of the NDRs:<br />
<br />
Reporting-MTA: dns; resqmta-ch2-01v.sys.comcas<wbr />t.net [69.252.207.33]<br />
Received-From-MTA: dns; resomta-ch2-07v.sys.comcas<wbr />t.net [69.252.207.103]<br />
Arrival-Date: Wed, 27 Feb 2019 20:05:41 +0000<br />
<br />
<br />
Final-recipient: rfc822; user2@domain.com<br />
Diagnostic-Code: smtp; Connection timeout<br />
Last-attempt-Date: Fri, 01 Mar 2019 06:24:14 +0000
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<div>
<blockquote>
so if a user on the comcast.net platform, emails<br />
<br />
1. old server<br />
2. new server<br />
<br />
it fails ?
</blockquote>
<br />
Yes, exactly -- the interesting part is that if all of the users in the email are on the old or the new (but not both) &nbsp;the email goes through. &nbsp;Very strange. &nbsp;There is only one MX record because there is only on IP address.
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<div>
how is email being delivered to your servers ?<br />
<br />
one single SMTP server for the domain (site)
</div>


<div>
Yes. Mail is routed to the Barracuda email security appliance and then to the exchange 2010 server. If the mailbox is not on that server it forwards to the old sbs server. <br />
<br />
I know this is working fine both in and out because I can send to both old and new together, from my gmail account and everyone gets it.
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<div>
Okay, I understand that the Barracuda email security appliance is the End Point in your organisation.<br />
<br />
So SBS2003 and 2010 in the same Exchange Organisation ?<br />
<br />
Therefore it does seem the fault lies in the COMCAST.NET.<br />
<br />
Is it one of their mail servers delivering mail (e.g. smart hosts) or client on their infrastructure using DNS lookup.
</div>


<div>
Yes, Andrew and there are no entries in the Barracuda logs for these user1+user2 emails. &nbsp;But if I send them individually, sure enough, they show up. &nbsp;This has been going on for about a week and a half now and Comcast rarely answers their phone.
</div>


<div>
if you want to send me your email addresses and domains privately, we can run some bounce tests here, with tool sets we use.
</div>


svillardi

<div>
Andrew, did you find anything?
</div>


<div>
<div class="content wysiwyg-content">
I am migrating an email server from SBS2003 to Exchange 2010. &nbsp;(I know it's old.) &nbsp;In testing co-existence with the old server, I have migrated 5 users from the old 2003 to the new 2010 server. &nbsp; These two servers are behind a firewall, with one external IP address. &nbsp;Migrated users are receiving email. &nbsp;Mailboxes on the old server are also sending and receiving fine.<br />
<br />
However, when someone from comcast.net sends an email to user1@domain.com on the old server and user2@domain.com on the new server, comcast.net never sends the email. &nbsp;I have tested this on their portal, and from Outlook. &nbsp;I have tested email from a gmail account which works fine.<br />
<br />
I am getting temporary and permanent connection errors from comcast.net.<br />
<br />
While I am trying to work with Comcast, is there anything I can do to determine what the cause of this issue is?
</div>
</div>


I am migrating an email server from SBS2003 to Exchange 2010.  (I know it's old.)  In testing co-existence with the old server, I have migrated 5 users from the old 2003 to the new 2010 server.   These two servers are behind a firewall, with one external IP address.  Migrated users are receiving email.  Mailboxes on the old server are also sending and receiving fine.

However, when someone from comcast.net sends an email to user1@domain.com on the old server and user2@domain.com on the new server, comcast.net never sends the email.  I have tested this on their portal, and from Outlook.  I have tested email from a gmail account which works fine.

I am getting temporary and permanent connection errors from comcast.net.

While I am trying to work with Comcast, is there anything I can do to determine what the cause of this issue is?

Comcast email not going to domain with co-existent 2003 and 2010 Exchange servers

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.

Outlook

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.