I finally (it's taken a while, although I've not been actively trying to "debug" the issue) figured it out:
You HAVE to enable "Send Referrer Information". Press F12 or go to the QuickPrefs and look for it.
Normally I turn it off since many times it's abused and a security issue, but for Amiga.org you must turn it on for pmail to work. Sucks and I don't know why, but that's the deal I guess.
BTW, when it doesn't work the pmail window says:
"Errors
None
Go back"
... yet it never sends the pmails.
DING DING DING DING!
I just realized: the referrer info sending is what's also keeping Opera from posting or editing posts! (just had it happen to me!)