This strange pop-up started occurring via Safari. Every time Safari was opened it would simply open the mail up and it kept re-opening every time you canceled. I couldn’t figure out which website was the offender. Regardless if you see phone number: 1-844-423-2465 (not affiliated with Apple) it is a good indication that your Safari has somewhat been hi-jacked.
Solution: Simple go to Settings > Safari > Clear History and Website Data. That will hopefully solve your issue.