You have to check if PHP mail is working in your server or debug WP’s mail, there are several ways to do this, I would first test if a basic PHP mail function works.
The is as simple as running this from a PHP file or the command line:
mail ('[email protected]', "Test", "Test mail from wordpress");
Find out what mail service is on your sever and debug it (sendmail, exim, etc, ?).
There are also some plugins that can help you debug email by sending test emails with custom headers on wordpress.org, as well as switching from PHP mail to SMTP (local or 3rd party).
To customize the activation email for multisite use the wpmu_signup_user_notification_email filter, for example:
function wp176598_new_user_notification( $user_id, $plaintext_pass="" ) {
$message = "your custom message";
wp_mail(
$user_email,
sprintf( __('Your login info for'), get_option('blogname') ),
$message,
$headers
);
}
add_filter( 'wpmu_signup_user_notification_email', 'wp176598_new_user_notification',11 );
Additionally there is : wpmu_signup_user_notification_subject for the subject line
More info: