No errors in the logs. I don't see any entry for default_process_limit in main.cf. I do see it in master.cf but it has no value, just a dash:
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smtp inet n - - - - smtpd
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Shouldn't it be like this with an entry for maxproc?
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smtp inet n - - - 100 smtpd
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I'm pretty sure I have never edited these settings, so I assume they are still from the stock install.