There's nothing to fix with your site because Screaming Frog is reporting them incorrectly since it is impossible for a
mailto: address to be broken because they are completely ignored by search engines, if the format is invalid or the email address does not exist then your email client would report so, not search engines or browsers.
You should contact Scream Frog and see if their developers can fix their software, otherwise, stop using their software if it's bugging you.
404's do not the equal broken site or bad rankings
It's often mistaken by Webmasters that 404's are somewhat of a bad thing, they are not, its a server response for a page that does not exist if a page doesn't exist then producing a 404 return status is an indication that your site and server are working correctly.
The only time you redirect 404s
The only time you ever want to redirect a 404 is when you have something relevant, for example, if
example.com/cats/ is deleted and you now have
example.com/felines/, then it would be perfectly reasonable to redirect cats to felines, but it would not be acceptable for both search engines and users to redirect
Nofollow on mailto links are pointless
There is absolutely no justification why you should be using nofollow on mail links, it's completely pointless.