Typically, if a user is opted out unexpectedly, it indicates they forwarded the NetCommunity message to another email address, then this email address opted the user out. For example, you send an email from NetCommunity to john@email.com. John forwards the email to jane@email.com. Jane doesn't want to receive the messages and proceeds with the Opt Out or email preferences links within the message. John is then opted out of the message. This is due to the fact that the opt out links in emails are unique to the intended recipient. In this case, John is the intended recipient.

Opting back in to email messages after unsubscribing