Override implicit conversion for existing types

Question

In my code I am handling emails sometimes as string string emailA = "[email protected]". But most of the time an actual MailAddress object is passed around and handled. My question is can I implement an implicit conversion from string to MailAddress if I have no control over either type?

If this was an object implemented by me i could use this: public static implicit operator MailAddress(string email) => new MailAddress(email);