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network administrator

Ariel Nicolas
Last Modified: 2017-12-02
Good evening!

I have a problem in my exchange 2007 server. The Error is "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..." Is  it possible that the cause of this problem is due incorrect ISP reverse DNS pointer? Currently, my DNS setup in DNS forward zone is servername.local and the reverse DNS pointer in ISP is also servername..local. I configured the send connector  using the ISP smarthost address which works after several weeks then after that the error repeated again that i cant send emails to other domain.  Is split dns can fix this problem?

Please help me how to resolve this problem.

Thank you in advance.
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J SSenior Systems Administrator

Are you receiving this when sending to any external domain?


HI! Jason,

Yes, I only received this error for sending external emails only. Right now, i'm currently testing the split dns but still no avail.
I'm not sure if i configure it correctly. I created a new zone mail.domain.com:  PrimaryZone  - Store the zone in AD >To all DNS controllers in this domain > mail.domain.com>Allow only secure then after that I created a new Host pointing to internal address. I transferred also the MX from .local zone to mail.domain.com zone then after that I restarted the DNS and Exchange transport service.

For additional info, Windows server is SBS 2008/Exchange 2007. I have two network cards. Pvt: Private Gateway/DNS: while in Public config : Public IPaddress/Subnet/ Gateway/ no DNS cofig.
Gareth GudgerSolution Architect
Most Valuable Expert 2014
Top Expert 2014

Typically when you get that error it is simply saying that your server can not connect to the recipients server. It might be DNS related, where your server can't resolve the mail recipients server. A quick test for this is to try an NSLOOKUP from your SBS box and see if you can locate and resolve the MX records for a few of your problem recipients.

It may also be network related. I would not recommend putting Exchange on two NICs like that. Much better to put SBS in a private network with a single NIC and behind a firewall/NAT device. We can test general connectivity issues by using TELNET.

A quick test is to run Telnet form your SBS box and see if you can connect to a few of the mail servers of the external domains you are trying to send to. See if you can do a telnet -o mail.domainname.com 25.

A good connection should could back with a 250 banner.
Peter HutchisonSenior Network Systems Specialist

Check for any other third party software such as Anti-Virus products or Firewall product that is blocking port 25.
Distinguished Expert 2019

How many ISP connections for you have? Does your firewall have a failover setup?
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Distinguished Expert 2019

You're quite welcome. And glad you're up and running.
Seth SimmonsLead Systems Administrator

No comment has been added to this question in more than 21 days, so it is now classified as abandoned.

I have recommended this question be closed as follows:

Accept: Ariel Nicolas (https:#a42105957)

If you feel this question should be closed differently, post an objection and the moderators will review all objections and close it as they feel fit. If no one objects, this question will be closed automatically the way described above.

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