Thanks, this is a possibility. However, that customer IP has been dealt with and hasn't been spamming for more than 60 hours at most (it's actually part of a dynamic DSL pool where port 25 outbound was supposed to have been blocked). Our problem appears to have started before the 27th.
Earlier today I noticed that the SPF record had "a" and "ptr" in it and changed it to "mx" only, as it should be in our case.
What I find most peculiar is none of these having problems:
1. Competitors over 10 times our size (similar % of gmail users), with invalid SPF records, no DKIM, who don't do *ANY* of what anyone here or off-list has suggested, who have PTRs for their DSL customers similar to ours (and didn't have a port 25 block), who've been blasting maintenance notices for years
2. Amazon, eBay, PayPal, Dell, etc.
3. Facebook with their ***user-generated content, in HTML***