451 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to Connect

Not open for further replies.

Cary Fields

I have this odd problem. Inbound and outbound email works great except a few problem domains. When sending email from our network to two particular domains they give me 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.
And after a few days, our sending user gets a failed delivery message. I tested using Gmail, and it seems to work fine to the same addresses/domains.
The two problem domains are: spectrumse.com & environcon.com
Both of which I can telnet to the smtp server from my CAS/HUB.
Any help would be appreciated.


Alexei Segundo

Does your CAS/HUB server connect directly to the remote Internet SMTP servers or do you have something at the Perimeter that handles your inbound/outbound SMTP traffic (e.g. Edge Server, Mimesweepeer, SendMail)? If you have a perimeter level server can you telnet directly from that to the remote SMTP servers?


Cary Fields

There is nothing on the Outbound side, its direct. Inbound, I have a Barracuda AV/AS appliance.

Alexei Segundo

Sorry Cary, I can't think why you should be able to connect using telnet on the same box, but the Exchange connection fails. If it weren't for that I would suspect that those domains that are failing are enforcing a reverse DNS lookup on the sending (i.e. your) IP address.


Dave Kawula - TriCon

Run your domain on mxtoolbox.com / I would also advise that it could actually be a problem on their end.

When performing your telnet testing are you doing a full test or just checking to see if your can hit port 25.

Most errors won't show until you get a little further in.

I run into issues like this all the time. If your mail server works for 95 % of outbound domains -- Odds are this:

They subscribe to a black list provider -- You are on the list

They use SPF Record Checking -- You haven't set them up

or they are using reverse lookups on your external mail alias -- and it isn't setup correctly.

If all of the above are correct -- Talk to their internal IT Exchange Admin and see if you have ended up on a spam filtering appliance on their and and ask them to whitelist you.

Hope this helps.


Daave Kawula Principal Consultant TriCon Technical Services Inc www.triconts.com dkawula@triconts.com www.insidetheregistry.com

Novak Wu

Have you installed Cisco PIX firewall or other firewall in your environment? If so, please refer to the following article to disable Mail Guard feature. For more information, please refer to the following articles:



Also, please navigate to the following location C:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue\mail.que and check the size of the file. Then, please refer to the following article to do offline defragment on mail.que.


After that, please try to send the email to the target domains again.

Hope this helps.

Novak Wu
Not open for further replies.
Thread starter Similar threads Forum Replies Date
T 451 4.4 dns query failed on exchange 2010 hub transport server Exchange Server Administration 1
C 451 5.7.3 cannot archieve exchange server authentication. Exchange Server Administration 4
K Exchange 2010 error 451 Open relay not allowed Exchange Server Administration 2
D 451 4.4.0 DNS Query Failed. Exchange Server Administration 3
M 451 4.4.0 DNS query failed. The error was: SMTPSEND.DNS.InvalidData; DNS returned an invalid respons Exchange Server Administration 3
M Duplicate Primary Mail Accounts outlook 2010 Using Outlook 0
G Auto accept meeting request for non primary account Outlook VBA and Custom Forms 1
P How to filter primary contacts in business contact manager BCM (Business Contact Manager) 0
Diane Poremsky How to remove the primary account from Outlook 2010/2013 New Slipstick.com Articles 0
JorgeDario ¿What property of mailitem can be used like primary key? Outlook VBA and Custom Forms 6
M Outlook 2012 Calendar Changing Secondary Calendar to Primary Calendar Using Outlook 1
P Can't fix my primary email account! Using Outlook 1
R Additional e-mail address converted to primary address? Exchange Server Administration 4
M Attempting to delete Exchange account in Office 2010 gives "primary account cannot be deleted" Using Outlook 8
G MS Office/Outlook 2010, when using a secondary inbox along with your default/primary inbox in 'Cashed Exchange mode' Using Outlook 3
J Mail window always restores back to the primary mail inbox Using Outlook 2
M How to remove a DAG but leave primary Exchange DBs intact Exchange Server Administration 2
B Default or Primary Exchange server? Exchange Server Administration 4
Z Exchange 2010 SP1 - Required DAG number of nodes for Primary and Secondary Data Center deployment Exchange Server Administration 2
K I want to start using outlook as my primary mail service. How can I use Outlook with my existing Yahoo & Gmail accounts? Using Outlook 1
D How to rename the display name of the primary outlook mailbox in Outlook 2010 / Exchange 2007 Using Outlook 1
M DR Failover Testing for a DAG in DAC Mode. Errors on Switch Back to Primary Datacentre. Exchange Server Administration 7
D Exchange 2010 - Update Address Policy - Wont update primary SMTP address Exchange Server Administration 3
A Assigning primary mail box in Outlook 2002 Using Outlook 1
N why the DAG address ( FQDN or IP ) always resolves to the primary Exchange Server in OUTLOOK? Exchange Server Administration 3
R Multiple Calendars - Change primary Calendar Using Outlook 5
S messages sent to non-primary email address Exchange Server Administration 4
G 'primary' filegroup is full BCM (Business Contact Manager) 1
B Primary file group is full error message BCM (Business Contact Manager) 4
W Primary Contacts and importing BCM (Business Contact Manager) 1
I header from changed to target recipient Exchange Server Administration 3
M Remove download or save target as option with attachments in OWA 2007 Using Outlook 3
D When I drag a message from my Inbox to another folder, the target folder disappears and only reappea Using Outlook 2
E The server you are connected to is using a security certificate that cannot be verified. The target Using Outlook 4
M VSTO_SUPPRESSDISPLAYALERTS not working on target PC Outlook VBA and Custom Forms 2
Similar threads