[Plugin: Cimy User Extra Fields] Which custom mail takes preference

I have both Theme- my – login and cimy-user-extra-field plugins installed. I also wrote a wp_new_user_notification function in my functions.php file. How is it decided that who will be sending my email? I thought anything in functions.php should take preference but its other way round 1st Theme-my-login and then cimy. How can I make function.php user_notification function take preference over the two without disabling

[initial code search possibly explains reason behind TML preference which removes the default function by hook but not for cimy and its module can be disabled so TML is not an issue as such but just wanted to confirm my understanding]