Unable to send external emails from Exchange server 2010

I am unable to send out any external emails from my exchange server 2010 all emails are stuck in queue. I have check my queue viewer and found all emails are stuck there and showing the error as per below.

451 4.4.0 primary target ip address responded with 421 4.2.1 unable to connect

451 4.4 0 primary target ip address responded with attempted failover to alternate host

Please provide me the solution on this. business communication impacting due to this.

THanks in advace
Walmik BoradeAsked:
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MAS (MVE)EE Solution Guide - Technical Dept HeadCommented:
HI Walmik Borade,
Welcome to Experts Exchange.
Do you have a send connector configured?
https://technet.microsoft.com/en-us/library/aa997285%28v=exchg.141%29.aspx?f=255&MSPPError=-2147217396
https://www.youtube.com/watch?v=dCPyk0_X894

Did you configure external DNS for external lookup?
https://technet.microsoft.com/en-us/library/bb123514%28v=exchg.141%29.aspx?f=255&MSPPError=-2147217396

Please check your IP is blacklisted or no
https://mxtoolbox.com/blacklists.aspx

Thanks
MAS
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Walmik BoradeAuthor Commented:
Hello Mas,

Thanks for your  replay. Please find my inputs as per below.

Do you have a send connector configured?
Yes

Did you configure external DNS for external lookup?
Yes

Please check your IP is blacklisted or no
Yes it is showing black listed in two places. can you please guide me how to remove the same.

Regards,
Walmik
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MAS (MVE)EE Solution Guide - Technical Dept HeadCommented:
You have to contact them and request to delist/remove from blacklist
Before that your have to make sure no SMTP traffic(spamming) goes out from that IP except from Exchange server.
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MaheshArchitectCommented:
The problem exists with all outbound external emails or for few?
how you setup send connector?

Is it using dns lookup on hub transport server or it is configured to forward emails to smart host?

Either you lost internet on exchange server or if smart host is failed / not working

OR

If problem exists with few emails, probably those domains have some issues with inbound connectivity, check if you are able to reach their MX from your exchange server, because error sounds that exchange unable to reach that MX....
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MAS (MVE)EE Solution Guide - Technical Dept HeadCommented:
If you have a firewall you have to block SMTP traffic for all IPs except Exchange server.
Create two rules in firewall to block SMTP traffic.order as below. In the below example Exchange server IP is 192.168.1.100.
source                      destination                 ACTION                                  SERVICES
192.168.1.100               WAN                          ALLOW                                   SMTP
LAN                                 WAN                         DENY                                       SMTP
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Walmik BoradeAuthor Commented:
I have remove the mail server IP address from black list but still emails are stuck in exchange queue. Can you please assist me how to deliver those emails.

Regards,

Walmik
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MAS (MVE)EE Solution Guide - Technical Dept HeadCommented:
What is the error you can see now in the Queue Viewer?
Please try restarting Transport service.
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MAS (MVE)EE Solution Guide - Technical Dept HeadCommented:
Enough information to confirm answer.
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