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Exchange 2010 SP3 Enterprise - Outbound Emails through 2 CAS servers

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Last Modified: 2018-05-10
Greetings,

We are running Exchange 2010 SP3 Enterprise on Windows 2008 R2 Enterprise.  4 Servers:  2 servers with CAS/Hub Transport role, and 2 servers with Mailbox Role.  Mailbox Servers are set up with DAG.
All 4 servers are at the same site/subnet.  No load balancing configured.
Something I'm trying to understand is with outbound email: 95% of the time emails get routed out through one CAS/HubTransport server and other times, maybe 5%,  route out through the other CAS/Hub Transport server.  I'm not sure what logic (or configuration) determines why emails get routed out one CAS/Hub Transport server vs the other.  Is this something native within Exchange that knowingly changes the outbound path for any particular reason?
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I believe you are correct with one server will take the bulk of the emails, and the other one will take the overload.  I seem to remember reading that somewhere too.  I'm strongly suggesting NLB for out environment, but the budget isn't there yet.  
The problem I'm having is one of our users is getting NDR's sending to one user at one domain when it gets routed through this 'secondary' CAS/Transport server.  It's getting rejected right away with error of a reverse DNS lookup failure for this servers external IP.  No issues when emails to this one domain gets sent through our 'primary' CAS/Transport server.  I'm not having any other bouncebacks for emails going through the 'secondary' CAS/Tranport server, just this one.  I've done all the nslookup, dns queries, telnet, etc., in troubleshooting.  It's always those 'one off's', right?  
Have you seen this behavior before?
Thank you,

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