What should you do when you see a spam rule violation indicating that the message only contains text/html MIME parts?

When faced with a spam rule violation indicating that the message only contains text/html MIME parts, the most effective action is to add a text version of your email. This approach directly addresses the concern that many email providers and spam filters have about emails lacking a plain text alternative.

Providing a text version improves accessibility for recipients who may not be able to view HTML content properly. It also assists spam filters by presenting a clear format that can be evaluated further for its legitimacy. Additionally, having both text and HTML versions enhances user experience, as some email clients may prefer one format over the other.

While the other options may improve your email's compliance or effectiveness in different contexts (such as providing an opt-out link for legal compliance or reviewing the subject line for relevance), they do not specifically tackle the underlying issue associated with the spam rule violation regarding MIME parts. Adding a plain text version directly corrects the content imbalance that triggered the violation in the first place, making it the right course of action in this scenario.