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Exchange 2010 mail flow issue (CAS servers)

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  • Exchange 2010 mail flow issue (CAS servers)

    Hi all, I'm facing this issue of mail flow issue. I will provide a brief description of our exchange architecture. Our office consist of two different domains.

    1. Dobrik.local
    2. Costos.local

    Dobrik CAS server
    1. DobrikCAS1 (
    2. DobrikCAS2 (
    3. DobrikCAS3 (

    Costos CAS server
    1. CostosCAS1 (
    2. CostosCAS2 ( )

    Setup by previous guys:
    1. DobrikCAS1 and DobrikCAS3 will have send connector pointed to DobrikCAS2.
    2. DobrikCAS2 will have receive connector to receive emails from DobrikCAS1/3
    3. On DobrikCAS3 Receiver connector, Authentication is set at TLS and Exchange Server Authentication. Permission is set at Anonymous Users, Exchange Servers and Legacy Exchange Servers.

    Issues faced currently:
    1. Users get connected to random DobrikCAS (e.g 1, 2 or 3)
    2. I'm getting user feedback to me saying they get bounced back emails stating "Generating Server: DobrikCAS1 (Sometimes DobrikCAS3) being rejected from DobrikCAS2 #550 5.7.1 Unable to relay ##"
    3. To fix that issue, I unchecked Exchange Server Authentication and enabled "Externally Secured", after which the emails seems to be able to go through. Right now, I'm getting another error saying "451 4.4.0 primary target ip address responded with: "451 5.7.3 cannot achieve exchange server authentication", so I enable back Exchange Server Authentication, again, the problem is solved.
    4. Question here is, from what I know, both CAS1 and CAS3 "route" email into CAS2 to send out. It contradicts one another as one time the problem says unable to relay, and another time unable to authenticate with Exchange Server. Both options checkbox cant coincide together.

    Is there anyway to indicate or force users to connect to a certain CAS server?

    What should I do? I'm out of options..
    Last edited by xnait; 21st April 2015, 05:03.