Although it's not fatal, I got annoyed by the messages of this type:
... sendmail[]: m2FJiKv7007674: Milter (bm-milter): write(D) returned -1, expected 97: Broken pipe
... sendmail[]: m2FJiKv7007674: Milter (bm-milter): to error state
There's a missing abort callback, and probably others, which means that Sendmail/Milter falls back to default error states, which is what that message is about, rather than it being handled by the Milter code itself.
The attached patch applies the missing abort callback (with a debug message which you can comment out if you like of course), and maybe only after this one of mine:
http://sourceforge.net/tracker/index.php?func=detail&aid=1910018&group_id=168582&atid=847324
It should make some error conditions at least slightly less noisy, but I can already see that it does not handle all cases.
Using this and the Sendmail::Milter perl module docs you should be able to quite easily add any other missing callbacks.
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File Added: bm-milter-1.0.5-missingcb.patch
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Replaced the patch version with an improved one that has a toggle switch for the error callbacks to be verbose.
revised again.
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File Added: bm-milter-1.0.5-missingcb.patch