Well it would be a popup similar to this,
https://vborg.vbsupport.ru/showthread.php?t=270070
Then in the message, the way I would like to see it, if feasible, is it asks, hey you, is this your email addy? If user clicks yes, it updates the DB with TIMENOW, so they don't get the message till the time you set is up. If they click no, then inside the same popup, there would be a field to enter your new email addy, anoce entered, user clicks submit, and their account gets updated with the new addy.
If the later is not possible, then it would be a link to their usercp.