Other than the obvious Outlook/Google calendar reminder I use Hetrix tools for uptime monitoring and they also offer domain and SSL cert expiration monitoring. My registrar will also email me before it expires.

I’ve used iRedMail hosted at home for the last year or so and love it. Even has ActiveSync integration for mobile devices. The key to selfhosting email IMO is to use a SMTP relay service to send outbound mail. I use SMTP2Go and it just works, even free. Everyone’s complaints about blacklists and reputation go away when you use a reliable relay.

Just for future reference, there’s no need to wait until expiration to move to a new registrar. The new registrar is required to honor the time left on your current registration. I’ve moved domains with years left and no issue.