delay in mail flow between exchange and ISA

Dear Experts,

I have an exchange 2007 server and a ISA server.  The ISA server has the necessary edge transport role installed and all has been working well for a few years now.

Lately, I have noticed that theres a delay in delivering incoming messages. In the queue viewer on the ISA server, I can see the messages in a queue which is bound for the exchange server. But I also notice a lot of strange undeliverable messages in there.

I have attached a screen dump for examples.  When I delete them, mail flows again.

I find it strange because its like the ISA box cant see the exchange server. DNS is functioning correctly.

Any ideas?

thanks
queue-on-ISA.jpg
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tech53Asked:
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Simon Butler (Sembee)ConsultantCommented:
http://exchange.sembee.info/2010/hub/filter-unknown.asp

That needs to be done on the Edge server. The agents should be already installed, so it just needs to be enabled.

Simon.
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Matt VCommented:
Are all the undeliverables coming from the same address or going to the same address?  I would start looking there.
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Simon Butler (Sembee)ConsultantCommented:
The content in the queues are NDRs.
Ensure that you are doing recipient validation on the server, so that it rejects email from non-valid recipients. It may well be that the server is delaying email because of the bogus email it has to process.

Simon.
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tech53Author Commented:
Thanks for helping. The emails have different recipients and senders. However simon has correctly identified them as NDRs - given the undeliverable subject.

Simon, how can I enable recipient validation?

Thanks again.
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tech53Author Commented:
thanks simon.  I've applied that and will monitor the queues.
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tech53Author Commented:
Simon,

That seems to have done the trick.

You've helped me before on a few occasions. Many thanks for sharing your wisdom.
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