[lug] Email brokenness?

[Chris]

Hi, everyone.

I've been having a really annoying time sending mail to users of
comcast and yahoo, and I suspect it's some antispam trick that relies
on something set differently from how it is.

I pull my mail to my desktop, and when sending mail, my client ssh's
to my mail server and sends it from there so that it appears to be
coming from the mail server rather than my desktop.  Anybody doing a
reverse lookup on my domain and checking the source IP should not find
any discrepancies.

Meanwhile, I'm getting errors like these when sending to users on
comcast.  Postfix on my mail server is failing, with this error:

<username at comcast.net>: host gateway-s.comcast.net[204.127.202.26] said: 550
    [PERMFAIL] comcast.net requires valid sender domain

What the heck is that supposed to mean, and what do I do about it?

--
    Chris Riddoch
epistemological humility