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LED 451 4.4.0 421 service not available (connection to blocklisted host (104.47.6.36 - DNSBL)) Exchange 2016

Benard Mwanza 1,001 Reputation points
2022-04-05T13:38:51.217+00:00

My exchange server 2016 is unable to deliver emails externally to internet.
Outbound mails are stuck in queue with the same error message displayed in exchange queue toolbox.

I tried the following fix without success:
Restart the server
Enabled TLS 1.2 both server and client registry keys for the server, restarted afterwards
Created a new send connector and assigned valid third party ssl certificate, restarted transport service
Tried to restart IIS.
All exchange server services are running
My DNS resolution internally is working properly
Third party certificate is bound to IIS & SMTP services.
Windows firewall is disabled.
No third party anti virus is configured in the server.
There no a single error event logged in win event logs under applications

Telnet diagnostics is off somewhere.
When i telnet localhost or the server its own IP address on port 25(telnet localhost 25 or telnet 192.168.x.x 25) I'm able to get a ESMTP banner, however when i try to telnet google smtp on port 25 or office 365 smtp(telnet smtp.gmail.com 25 or telnet smtp.office365.com 25), i get an error, that Connection to host lost. with additional message ....,421 service not available (connection to blocklisted host (104.47.6.36 - DNSBL)),

The smtp send connector keeps logging the same error repetitively.
This is the error. What is wrong?

`#Software: Microsoft Exchange Server

Version: 15.0.0.0

Log-type: SMTP Send Protocol Log

Date: 2022-04-05T11:01:57.626Z

Fields: date-time,connector-id,session-id,sequence-number,local-endpoint,remote-endpoint,event,data,context

2022-04-05T11:01:48.383Z,Internet,08DA16F263D3E30F,0,,192.168.x.x:25,,,attempting to connect
2022-04-05T11:01:51.186Z,Internet,08DA16F263D3E30F,1,,192.168.x.x:25,,,"Failed to connect. Winsock error code: 10060, Win32 error code: 10060, Error Message: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 192.168.x.x:25"
2022-04-05T11:02:12.560Z,Internet,08DA16F263D3E311,0,,104.47.8.36:25,,,attempting to connect
2022-04-05T11:02:12.771Z,Internet,08DA16F263D3E312,0,,104.47.6.36:25,,,attempting to connect
2022-04-05T11:02:13.975Z,Internet,08DA16F263D3E312,1,192.168.x.x:8612,104.47.6.36:25,+,,
2022-04-05T11:02:14.483Z,Internet,08DA16F263D3E312,2,192.168.x.x:8612,104.47.6.36:25,<,421 service not available (connection to blocklisted host (104.47.6.36 - DNSBL)),`

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  1. Kael Yao-MSFT 37,676 Reputation points Microsoft Vendor
    2022-04-06T01:45:58.517+00:00

    Hi @Benard Mwanza

    This error message indicates your public ip address may be on some blacklists so the target servers refuse to connect.
    You may use some online tools for example mxtoolbox.com to have a check.
    (Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

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