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Exchange 2003 NDRs

Perhaps someone can help / explain what is going in. We seem to be stuck in an infinite NDR loop. I know I could create a postmaster@domain.com address and all would be sorted, but that doesn't explain how this could be overlooked.

Our server received spam from email@other.com (which doesn't exist) to postmaster@domain.com (which doesn't exist). Our server sends an NDR to email@other.com, which replies with an NDR to postmaster@domain.com, which replies with an NDR to email@other.com, rinse and repeat.

Unfortunately I'm not actually at the server, so cannot create the postmaster@domain.com address right now, which means an email every ~1-3 seconds is being sent and received, constantly. That is obviously no good for the server's disks.

Now, assuming I create postmaster@domain.com, what is to stop email@other.com sending a mail to a454txaw4awxd4t@domain.com and the whole process starting again? Yes, I could block *@other.com (or the IP), but then what about email@them.com (different domain, different IP), or ac346tsd45sd5@q3d46sd5ts45t.com? I obviously cannot block *.com.
2 Solutions
This should be helpful

Exchange queues fill with many non-delivery reports from the postmaster account in Small Business Server 2003
VampireofdarknessAuthor Commented:
I had thought I had enabled sender filtering, connection filtering, etc... but apparently they had been disabled.

Re-enabled in order for the 'reject recipients not in directory' option and all is well.

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