I learned today of a well-known software vendor whose business has suffered as a result of poor list management practices. It's not the first, and probably won't be the last. This sorry tale only goes to illustrate the importance of avoiding becoming an inadvertent spammer.
It appears that, although it had been legitimately sending mailings to its customers, the vendor had been ignoring unsubscribe requests. As I've said before, any unwanted bulk email sent by an organization after an appropriate unsubscribe request is spam -- an organization that fails to act on unsubscribe requests in this way is a spammer.
As a result of its failure to honour unsubscribe requests, complaints about the spam began to accumulate at the feet of the various organizations that track spammers' activity. Crucially, these include sender reputation services, such as DNSBLs (also known as IP blacklists). Inevitably, despite the fact that the majority of email it sent was legitimate, the vendor gained a negative reputation as a spammer.
This caused some recipients of its email to reject or otherwise filter these legitimate messages. Not only were legitimate direct marketing messages filtered, but also messages containing customers' license keys, technical support replies, etc.
This is indeed a cautionary tale: the lesson for senders is that the unsubscribe process is truly a mission-critical part of your direct marketing or transactional email workflow. Failure to ensure its integrity can not only cause legal problems, but damage your customer relationships and your business.