The HML merge field that should be included in an email to allow prospects to manage their email preferences is {{OptOut}} or {{EmailPreferenceCenter}}. The {{OptOut}} merge field inserts a link that allows prospects to opt out of receiving emails from the sender. The {{EmailPreferenceCenter}} merge field inserts a link that allows prospects to choose which types of emails they want to receive from the sender. The {{Unsubscribe}} merge field is not valid in HML, as it is only used in PML. The {{ReportSpam}} merge field is not recommended, as it can negatively affect the sender's reputation. The {{Opt_Out}} merge field is not valid in HML, as it uses an underscore instead of a camel case1. Reference: 1: HML Merge Fields