Mails are another interesting factor. If you use your own domain for mails, where should you have the nameserver for that domain? If I use that mail to sign up

Mails are another interesting factor. If you use your own domain for mails, where should you have the nameserver for that domain? If I use that mail to sign up for a registrar and the service disrupts, theoretically my mails shouldn't work anymore as well and I would be unable to recover anything.