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What does this bounce error mean: "550 IP Authorization check failed - psmtp"

Under "EMAIL REPORTS" I see some emails bouncing with the reason "550 IP Authorization check failed - psmtp". What does that mean?

Some emails to those same email addresses do not bounce, but some do.

I ran the offending email through by sending it BCC to and the report looks clean:

SPF Check : pass
Sender-ID Check : neutral
DomainKeys Check : pass
DKIM Check : pass
SpamAssassin Check : ham

I posted the full report here:

Any ideas on what I can do to avoid the bounces?
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