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https://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_dep365/dnsconnectordelivery-queue-stuck-connection-timed/44cdbf12-4d84-4ba6-aae2-1ba08b0bf68f
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https://www.experts-exchange.com/questions/27899268/DnsConnectorDelivery.html
451 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to connect. Now, there could be loads of reason why the recipient domain might reject connections, cause you may not have reverse PTR, OR your IP isn't recognized (dynamic IP sorto) etc etc, what you'll need to check is if you can send emails to various email domains and conclude if your setup is working or not.
https://community.spiceworks.com/topic/282192-dnsconnectordelivery
Dec 14, 2012 · "451 4.4.0 Primary target IP adressess responded with: "421 4.2.1 unable to connect." I can telnet the destination on tcp25, I can ping the IP. They are also whitelisted in the firewall.
https://social.technet.microsoft.com/Forums/exchange/en-US/36d00792-efcb-49d3-a9a0-69d6a36a4ab2/451-440-primary-target-ip-address-responded-with-quot421-421-unable-to-connectquot
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https://practical365.com/exchange-server/messages-queuing-error-451-4-4-0-dns-query-failed/
Oct 16, 2014 · Thank you for this… We just started experiencing this with accounts that we migrated from our on-premise Exchange 2010/Server 2008 environment to Office365, where we are routing our mail first through our on-premise server and then to O365.
https://www.urtech.ca/2011/12/solved-4-2-1-unable-to-connect-attempted-failover-to-alternate-host/
Dec 21, 2011 · 451 4.4.0 Primary target IP address responded with: “421 4.2.1 Unable to connect.” Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts. Below are the three items you need to check: Test the connection by sending an email through command line.
http://azuredummies.com/2016/04/30/exchange-2013-421-4-4-2-connection-dropped-due-to-socketerror/
Apr 30, 2016 · This saved my for Exchange 2016 problem after installing updates. Not sure how or but after 6 hours of testing various things, this worked! I realize this thread is old, but if this helps someone else in this problem with the help or Google searching for Exchange 2016, then so be it!
https://markgossa.blogspot.com/2015/09/451-440-primary-target-ip-address.html
Sep 15, 2015 · An application on the server may be blocking the connection. This is commonly misconfigured antivirus software. An example of this is McAfee Access Protection where, if misconfigured, doesn't allow outbound SMTP traffic.
https://alittleofnothing.wordpress.com/2014/09/22/exchange-online-office-365-hybrid-configuration-connectivity-problems-must-issue-a-starttls-command-first/
Sep 22, 2014 · Exchange Online Office 365 Hybrid configuration connectivity problems – Must issue a STARTTLS command first. We noticed that we were not sending any Email from our On-Premise Exchange Server to the Exchange Online Server so I checked the queue. In the EMC go to the Toolbox and click on Queue Viewer.
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