Fun with email
Postoffice, from almost the beginning of time (March 15, 2004), has used Evan Harris's greylisting scheme to cut down on the amount of spam that shows up in mailboxes on pell. For most of the time it's had greylisting, relatives have been able to (at least when they're not blocked by the ever-changing greylist) send mail to me, even if it's occasionally delayed a few hours by the greylist.
Last week, relatives who use cableone discovered, to their (and my) annoyance, that cableone has apparently "upgraded" their mail software to one that treats a 451 (temporary failure; local error) as a 5xx (permanent failure, don't retry.) This has the unsurprising side-effect of ensuring that mail sent through new and different copies of the mail server will bounce, even if the client MTA is just telling it to cool its jets for a little while.
Perhaps it's because I'm polite and return a 451, instead of simply spitting out a 421 (hey! I don't like you! Go away! [pulls plug and dies]) and disconnecting. Maybe I'll try that method tonight.
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And I did make the change, and rolled postoffice over to using it, and lo and behold!, the stupid cableone mail servers started being able to deliver mail again. So the lesson is that when you want to greylist someone, there are mail servers out there that you can't greylist without having the server commit ritual suicide.
Grrrr.