Everyone gets lead-gen email at some point. Here's how you can turn that lead-gen email back on whoever sent it to you.
Dear (fill in name),

I wanted to contact you because one of my colleagues recently received an email containing an ad for XXXXXX, but he did not sign up to receive such mailings. My guess was that XXXXXX didn't realize that this mailing was seemingly going to some people who hadn't signed up, and that the partner company was sending the mailings to people who didn't want them. I don't know if this is considered spam or not, but I thought I should get in touch with you so that your company could look into it to confirm that the mailings are being sent only to those who want them.
By the way, we offer exclusive newsletter ads and dedicated mailings to our 100% opt-in only subscriber list. Our list has been built in-house over the last 10 years and is made up almost exclusively of active investors and people interested in financial products and services. We have four newsletters: XXX, YYY, and ZZZ, that all together reach over 90,000 people. Our exclusive text ads appear near the top, right under our table of contents. Our dedicated mailings are only sent to our opt-in subscriber list, and they are sent on a limited basis so our advertisers' messages get special attention from our audience. We're able to do HTML and text, please enquire for our rates because we do offer special discounts.