Quoting Simon Hobson (linux@???):

> That's the important thing to look for - and my money is it's related to SPF and/or DMARC.

It won't be SPF.

My domain has a strong SPF policy:

:r! dig -t txt linuxmafia.com +short
"v=spf1 a mx -all"

...and no mailing list post from me or my users violates it.

It'll be DMARC. (Long experience says.)