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LED=451 4.4.397 Error communicating with target host 421 4.4.2 connection dropped due to TimedOut RRS feed

  • Question

  • Hi there.

    Exchange 2010 on-premise and Exchange 2016 in AZURE.

    We are in phase of staged migration of mailboxes on EXCHANGE2010 to Exchange Online where we have EXCHANGE2016 configured in minimal hybrid as we used "as a proxy" for migration to Exchange online.

    We are experiencing mail flow issue. Current dns configuration is that inbound mail from internet goes thru EX2016 first and then to EX2010; for outbound mail flow we use EX2010 because EX2016 has blocked 25 traffic in AZURE.

    Issue:

    Mail flow works ok for some time, but then after random time the mail flow stops working with error:

    LED=451 4.4.397 Error communicating with target host 421 4.4.2 connection dropped due to TimedOut

    The only thing that helps is restarting both Exchange servers, then mail flow works ok (for some time).

    Any suggestions?


    bostjanc


    • Edited by B_C_R Friday, September 7, 2018 8:36 PM
    Friday, September 7, 2018 8:34 PM

All replies

  • Hi,

    Did you get the error message from the NDR? If so, when did you get the error? When you send the emails to internal users or to internet?

    How did you configure the outbound mail flow part (Connectors and DNS records settings)? Please make sure 25 port is open for the outbound mail flow in Exchange 2010 server. Besides, check if any network issue may cause the "TimedOut" error.

    Similar link: 421 4.4.2 Connection dropped due to TimedOut.

    Regards,

    Manu Meng


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    • Proposed as answer by Manu Meng Friday, September 14, 2018 9:11 AM
    Monday, September 10, 2018 7:08 AM