add_action( 'arm_after_send_email_notification', 'your_function', 10, 5 );
function your_function($from, $recipient, $subject, $message, $attachments) {
    //Do your action.
}

add_action( 'arm_before_send_email_notification', 'your_function', 10, 5 );
function your_function($from, $recipient, $subject, $message, $attachments) {
    //Do your action.
}

add_filter( 'arm_change_advanced_email_communication_email_notification', 'your_function', 10, 3);
function your_function($content, $user_id, $user_plan) {
    //$content Email Content
    //$user_plan Plan id
    //$user_id User id
    $content = str_replace('{SOME_VARS}', "Some Value", $content);
    return $content;
}

add_filter( 'arm_change_verification_email_notification', 'your_function');
function your_function($message) {
    //$message Email Content
    $message .= ' If you have any query, please contact site administrator.';
    return $message;
}

add_filter( 'arm_change_registration_email_notification_to_user', 'your_function');
function your_function($message) {
    //$message Email Content
    $message .= ' If you have any query, please contact site administrator.';
    return $message;
}

add_filter( 'arm_change_registration_email_notification_to_admin', 'your_function');
function your_function($message) {
    //$message Email Content
    $message .= ' If you have any query, please contact site administrator.';
    return $message;
}

add_filter( 'arm_change_email_content', 'your_function');
function your_function($message) {
    //$message Email Content
    $message .= ' If you have any query, please contact site administrator.';
    return $message;
}