Ignoring what was very important information for them
Email may be older than the web, but it's still one of the best ways of keeping in touch with your users, readers, customers etc. But, much like everything else on the web, it tends to get abused. Spam is obvious and most email services are able to keep out most of it.More problematic are unwanted, but legitimate emails. Many people sign up to all sorts of newsletters and email updates, knowingly or not and then have their inbox flooded. Most serious businesses and sites offer ways of unsubscribing, even if they are less than visible or obvious.
And those that don't should seriously start to consider it. Flooding users with email is one sure way of getting them to ignore it. And that's unfortunate if you may have something very important to communicate to them at one point.
Something like, say, the fact that millions of passwords have been leaked and that users should change them imediately as LinkedIn was recently forced to do. But over 4 percent of those that got the email marked it at spam since LinkedIn tends to overwhelm people with mostly unsolicited email.