Intermittent DNS Errors - bounce backs received on emails

Hi Everyone:

We are experiencing intermittent errors (very frustrating) with email that seemed to have started yesterday.
Emails being sent to domain fail intermittently showing up as DNS errors.  It works and then it doesn't work.

Error received is as follows:
Sorry, I couldn't find any host named (#5.1.2)

The nameservers are with Netfirms ( and and nothing has changed on the settings in several years.
I have contacted Netfirms to see if any issues on their end, and of course, they indicated that their dns servers are fine.
Having said this, when I check the domain on it shows the correct DNS entries but if I refresh the tool, every so often I get an error that shows: "No records exist" when reporting from ( always show the correct dns records).

I'm not sure where to go from here.  Netfirms (after 5 or so calls to them) says nothing is wrong on their end.
What other tools would you suggest I use to diagnose this?
Or what would your next step be?

I'm hoping someone out there can help as I've been on this since last night and am running out of ideas, short of moving the domain to a different registrar and name servers; however, that is my last resort!

Thanks for your feedback.

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Will SzymkowskiSenior Solution ArchitectCommented:
Intermittent issues like this is usually caused by Reverse PTR record not pointing to

I would start their first.

TheSonicGodAuthor Commented:
Hi Will,  
Thanks for your reply.  We have a PTR record pointing to
I suspect it may be one of the nameservers ( causing issues but I can't seem to prove it to Netfirms.

Any other suggestions?

Will SzymkowskiSenior Solution ArchitectCommented:
What is the exact error message you are getting on the bounce back emails?

Also have you checked the email headers as well for more info?

HariomExchange ExpertsCommented:

Does your exchange server uses for External DNS lookup ? , if so for testing purpose you can use google puble ip address , for external DNS lookup and check for issues.

after changing public ip if everything is fine then it may be one of the nameservers ( causing issues .

External DNS Lookup setting is available on Hub Transport Server Properties.
Also make sure "Use the External DNS Lookup settings on the transport server " is checked on Hub transport properties.
WORKS2011Austin Tech CompanyCommented:
what does and report?

What server, SBS, standalone server with exchange separate?

DHCP on your router or server?

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