This filter function creates a variable from Usermeta to be used with PMPro emails.

pmpro-usermeta-in-email-data.php

/**
 * This filter function creates a variable from Usermeta to be used with PMPro emails.
 *
 * The function returns a variable prepared for the special html syntax within PMPro emails. In this example, the value of the array key 'favorite_color' is returned and can be used in emails with the html placeholder !!favorite_color!!.
 */
function pmpro_usermeta_in_email_data( $usermeta, $email ) {
	global $current_user;
	$usermeta['favorite_color'] = get_user_meta( $current_user->ID, 'favorite_color', false );

	return $usermeta[0];
}
add_filter( 'pmpro_email_data', 'pmpro_usermeta_in_email_data', 10, 2 );