The most recent has the subject: EXCLUSIVE PARTNER OFFER: Create Beautiful Computer Generated Art Now with 50% OFF

It's reply to address is Special Renderosity Partner Offer newsletter@postoffice.renderosity.com

this is just the most recent. I'd say at least 1 of these each week for the last couple of weeks. It could be that they aren't really regular, but I know I've received a handful over the last few months.