Dear HN person who ask for my help

Fairly recently I received an email from someone here on HN. It was polite, referenced a thing I'd submitted, and pointed me at something I might be interested in. They finished by saying:

"Please feel free to give me any comments about it and to share it as you please."

So I had a quick poke about, then spent some time composing a reply, and sent that off.

The result (address redacted):

    A message that you sent could not be delivered
    to one or more of its recipients.  This is a
    permanent error. The following address(es) failed:

    xxxx@xxxx.xxxx 
    host mxa.mailgun.org [34.160.63.108]
    SMTP error from remote mail server after
    RCPT TO:<xxxx@xxxx.xxxx>:
    550 5.7.1 Relaying denied

    Action: failed
    Final-Recipient: rfc822;xxxx@xxxx.xxxx
    Status: 5.0.0
    Remote-MTA: dns; mxa.mailgun.org
    Diagnostic-Code: smtp; 550 5.7.1 Relaying denied

I don't know if this is my problem or theirs ... I suspect theirs.

This is not the first time this has happened. People, if you're going to ask someone else to spend their time helping you, make sure things work.