Expiry date user notification

WP has its own email function — wp_mail(). It’ll send any message to any email address. You need to be able to get all certificate references that are about to expire. I’ve no idea how that is being handled, but a DB query should be able to get the data, which should be able to be referred back to a user and their email.

You can schedule automatic tasks with wp_schedule_event(). The task can make a query for certificates nearing expiration, get the related email address for the user, then email a notification to them. Once set up, it will all happen automatically. The event trigger relies upon someone visiting your site for any reason. If no one visits, the scheduled task cannot run. This is rarely a problem for most sites, but something to be aware of if you have a low traffic site.