save_post action only when creating a new post

I have a Custom Post Type named Task. I created a function that sends an email to the selected Agent, notifying a new task has been assigned. Here is the function:

function real_estate_send_mail_to_agent() {
    global $post;

    // If this is just a revision, don't send the email.
    if ( wp_is_post_revision( $post->ID ) ) {
        return;
    }

    // Exit function if post type is not equal to task
    if ( $post->post_type !== 'task' ) {
        return;
    }

    // Email header
    $headers .= "MIME-Version: 1.0\r\n";
    $headers .= "Content-Type: text/html; charset=ISO-8859-1\r\n";

    // Recipient
    $agent = get_field_object("agent", $post->ID);// Get agent object from user_table
    $emailTo = (string) $agent['value']['user_email']; // Get agent email

    $agent_display_name = $agent['value']['display_name']; // Get agent display name

    // Email Subject
    $subject = "New Task: " .wp_strip_all_tags(get_the_title($post->ID));;

    // Email Body
    $message = "Hi <b>".$agent_display_name."</b><br/>"
    $message .= "You have been assigned a new task <br/>";
    $message .= "Please have a look at it ".get_permalink( $post->ID );

    // Send the mail
    wp_mail( $emailTo, $subject, $message, $headers );
}
add_action('save_post', 'real_estate_send_mail_to_agent', 11);

The function sends the email to the agent which is fine. The issue is that it triggers even when the post is updated or move to trash.

I want it to fire only when a new post is created using the save_post. The reason I need to use save_post is because I have to get the agent email from the user object in the User Field Type using the ACF plugin. If I use publish_post, the email is not sent, since it can’t fetch the agent email. Help please.

The save_post action also passes three parameters to your callback, one of which being $update which denotes whether the post being saved is an existing post or not.

/**
 * Save post metadata when a post is saved.
 *
 * @param int $post_id The post ID.
 * @param post $post The post object.
 * @param bool $update Whether this is an existing post being updated or not.
 */
function save_post_callback( $post_id, $post, $update ) {

    if ( $update ) {
        return;
    }

    //business logic...

}

add_action( 'save_post', 'save_post_callback', 10, 3 );

See:

• https://codex.wordpress.org/Plugin_API/Action_Reference/save_post