From the milter-null site
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A DSN message is automatically generated and sent to the sender of a message by a mail server in response to some error, like when the recipient address does not exist, mail box is full, message rejected, etc. When a spammer or virus sends mail often many of the recipient addresses are invalid, which often happens when guessing a large number of recipient addresses (dictionary attacks). This results in lots of useless DSN messages being generated, which is called "backscatter".
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But I don't fully agree with this, an MTA shouldn't simply send DSNs. I also note that Sendmail doesn't by default (which is good)..
So I suspect you are using Sendmail for relaying?
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Your MX servers should reject email for unknown users at the SMTP initial transaction and NOT forward them to internal SMTP servers without a "user check".
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This is how we configure ours, in fact some recent data I gathered:
Based on the last 24h, our figures show we drop (SMTP) 84.5%
Delete 2.98% from the spool
Deliver as identified as spam 0.67%
And deliver only 11.8% authentic email
Lea