Quoting russell@coker.com.au (russell@coker.com.au):
45x isn't a hard fail, all the 4xx codes are for temporary failures. 45x are for corrupted mailbox, server connection problem, or server storage limit exceeded.
Yeah, as I said, it was late (here), I was very tired, and I tried to remember the SMTP spec details from fatigued memory rather than bothering to check. Surely you knew I meant 55x, though. But aspies have to aspie? ;->
It's trivial to test, put in an NS record for a subdomain of one of your domains that points to a system that's well firewalled and refuses to respond to port 53. Then send mail.
Yeah, that, for example.
http://wiki.gandi.net/en/glossary/glue-record
The above page has a good description of glue records.
Jesus, Russell. I've only been teaching people about glue records for about a quarter century. As I said, I was overdue to go to bed and had no business attempting to work through technical problems.