Sam,
I agree that this particular ISP isn't my problem, however I wonder if courier tries more than the first three MX records even if they have problems such as pointing to hostnames that don't resolve to A records or having an IP address in them.
MX records that resolve to a non-existent hostname are simply ignored. Broken MX records that hold an IP address always result in a rejection, notwithstanding any other MX records.
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