- The service-based economy means that entities (individuals and businesses) have numerous relationships. For the typical individual the number of password-based accounts crossed the 100 threshold years ago, at a doubling rate of every 2--3 years.
- Responsibilities can be transferred. The person who signed up for your service 5 years ago may no longer be at the company.
- List purging is a Real Thing. A few years back I'd worked for an organisation that had ... numerous relationships ... with individuals and corporations. These received regular email messages. Nominally, requested. Included amongst these was a major Wall Street financial firm whose implosion years earlier hit lead news and headlines worldwide. Despite not existing for years, there remained hundreds if not thousands of addresses being sent email on a regular ongoing basis.
- Mail can be forwarded. It's quite possible that you're sending mail to one address that is is being forwarded, manually or automatically, to others. This raises issues in unsubscribe requests, but might at the least be an opportunity to reach out to your customer to clarify the situation.
- I don't know if revisiting email contact approval on a regular basis (say once every year or two) is yet a recommended practice, but I'd strongly suggest that it be so.
Your hat may be less blisteringly white than you presume.