Co-registration, which is also referred to as co-reg or coreg, allows subscribers to sign up for multiple email programs through one form.
A subscriber opting in to a popular fitness email program may also be presented with the option to sign up for another company's nutrition newsletter on the same form. The subscription information is then forwarded to the company sending the nutrition newsletter.
This affiliate marketing strategy can provide a way to collaborate to expand visibility and grow your subscriber base if done responsibly. However, in order to make more money, many co-registration partners combine multiple subscriptions in to one "catch-all" option and don't specify from whom or what email the subscriber will receive.
This form of email address acquisition often leads to high complaints because the subscriber does not recognize the sender and thinks the email they send is spam.