#5.5.0 smtp;550 all relevant MX records point to non-existent hosts

We are having on and off mail problems. We have a mail server (Exchange 2000) in our domain that is receiving mail from an other mail server in the domain of our Headquarters. Our primary mail address is in the HQ domain and mail send to that address is forwarded to our Exchange server.
This worked for a long time with no problems.

Now we are getting messages that mail is being rejected with the following NDR:

 #5.5.0 smtp;550 all relevant MX records point to non-existent hosts

When they try two minutes later the mail goes through without problems.

Where do I have to start looking for configuration errors? It puzzles me that it is working 99% of the time and 1% not. Configuration is either right or wrong...or is it???
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EVeugerAsked:
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SembeeConnect With a Mentor Commented:
That looks like a DNS issue. The sending site has issues resolving the lookups. It could be something as simple as a slow server, the lookup times out. Try again a minute later and it works because the result is cached.

Simon.
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