[lobster_johnson]

Some of the points about phone numbers also apply to email addresses.

For example, as has been shown repeatedly, most email validation patterns are simply wrong, and are best trimmed down to something like /^[^\s]+@[^\s]+$/.

Arguably more useful than a text-based email validation is validating whether the domain part has a valid MX. This will guard against many typos.

Another less-obvious fact that people eventually come to realize is that emails don't last for ever. I run a site where people constantly contact support about being unable to log in, because their email address is no longer active, and they forgot their password. (Some of them even forgot what their user name was.) It turns out many people use their work or school email, and that this email ceases to work after they leave. It's a good idea to regularly ask people whether their email is still valid.

All of the above could easily be bundled as a SaaS service. We use Mailgun for email address validation and for reading bounces (so we can alert users that their email is no longer working, in case they're stilled logged in), but a high-level, all-in-one service could be useful.