smtpd doesn't start

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I'm faced with a problem on the stable. The moment a network interface
goes up smtpd cannot be started. I mean, it hangs and gives no output,
neither with -dv or -Fv options, it even hangs with -n. Without a network
it still works as expected. I changed no related config, I believe. How
could I debug (and even better fix) that?
Thank you.