The majority of mails in my spam folder is plaintext mail. Second place are the extremely malformed HTML mails. Almost any wellformed HTML mail I receive is solicited at minimum.
I can recommend using the unsubscribe link if you forgot to uncheck the newsletter checkbox during registration. Alternatively send them a mail to A) remind them of the SPAM-CAN act or your local equivalent legislation and B) that they should delete your mail from the newsletter. That works almost always, if not you can always threaten legal action (where I live, that's an easy 600€ of profit in a courtroom).
I can recommend using the unsubscribe link if you forgot to uncheck the newsletter checkbox during registration. Alternatively send them a mail to A) remind them of the SPAM-CAN act or your local equivalent legislation and B) that they should delete your mail from the newsletter. That works almost always, if not you can always threaten legal action (where I live, that's an easy 600€ of profit in a courtroom).