[jackalope]

It's extremely unlikely that spoofed headers or the lack of an SPF record would get you listed on an RBL, especially Spamhaus. I can't guess what happened in your case, but somehow your IP address obtained a bad reputation or was unlucky enough to be in a tainted block. FWIW, the very first thing I do after getting an IP allocation is run an RBL check on it and demand a replacement if it's listed anywhere.

[jiggy2011]

Yes, it was a fairly weak hypothesis. OTOH we got on several RBLs a number of times and managed to get taken off them. Once I added an SPF record it hasn't been a problem since.

Didn't use SPF to begin with because there was a large number of hosts legitimately sending mail for the domain and it was a pain to get all of the IP numbers for various crazy reasons.