If you're running a serious mail service your IPs don't change that much. Not only will you have a pool of IP addresses just for that reason, you'll also likely need a few unused ones for when one of your users tanks your reputation and you have to roll them.

Nobody taking real money for email will use a randomly assigned IP as source. (unless it's randomly selected and then tested for reputation before use) Mailgun and other services even allow you to book a dedicated IP just for your outgoing emails.