First of all, you should know the difference between spam and email that is not spam.
Most people find opt-out email to be offensive and classify it as spam.
If you are sure you did not, then it may be someone attempting to appear to be a representative of the company in question, but who actually does not have the consent of the company. The main domain-name registry, Network Solutions, is a noteable example of this. It has in the past sent outright spam and has been widely criticized for this action among spam-fighters. In any case, if you are sure that you did not request the email, then you are well within your rights to report it as spam. Often, people receive email warning them of a dire threat due to new viruses or offering amazing rewards for continuing a chain letter.