[This story is not Perl-related, but it has a surprising solution to a perplexing technical problem]
Recently I switched webhosts for my various personal domains (including the YAPC::NA 2006 videos). Before switching, I splurged and purchased accounts on a couple of discount webhosts so I could try them out and compare for real instead of on paper.
After settling on one and moving my online presence there, I discovered that any email I sent from one particular domain to one particular friend bounced. I methodically ruled out spam filters and blacklists, but I kept getting the same error:
Diagnostic-Code: X-Postfix; host mail.example.com said:
550 5.1.0 <chris@domain-deleted>: Sender address rejected: User unknown
in virtual alias table (in reply to RCPT TO command)
What confused me was that the *sender* was rejected, not the recipient.
I finally realized that my friend has his domain hosted on one of the ISPs that I had tried and rejected. They still had an entry for my domain in their system, so they were checking *locally* to see if I was a valid sender. Since I didn't get around to setting up "chris@" for that domain before rejecting the ISP, it was not a valid email address and delivery was refused.
The solution: cancel my account on the ISP I rejected and wait for them to remove my entry from their DNS servers.