// onAuth function passed to smtp-server via options:
    onAuth(auth, session, callback) {
        // Supported authentication types
        if ( auth.method != 'LOGIN' && auth.method != 'PLAIN' ) {
            return callback(new Error('Invalid authentication type.'));
        }

        if ( auth.username == null || auth.username == undefined ) {
            err = new Error("Must provide a ussername");
            callback(err);
        }
        if ( auth.password == null || auth.password == undefined ) {
            err = new Error("Must provide a password");
            callback(err);
        }

        console.log('onAuth(): Using AUTH ' + auth.method.trim().toUpperCase() + ' to authenticate ' + auth.username.trim());
        log.info('onAuth(): Using AUTH ' + auth.method.trim().toUpperCase() + ' to authenticate ' + auth.username.trim());

        if ( authenticateUser(auth, callback) != true ) {
            return callback(new Error("Invalid username or password."));
        }

        return callback(null, {'username': auth.username.trim(), relay: true, ts: Date.now() });
    },







// authenticateUser(auth, callback)
//
// Helper functions for the SMTPServiceOptions object.
//
async function authenticateUser(auth,callback) {
    try {
        const username = auth.username.toString().trim();
        const password = auth.password.toString().trim();

        if ( !username || !password ) {
            return callback(new Error("Missing username or password"));
        }

        // FILTER:
        FILTER = { 'username': username };

        await mailboxes.findOne( FILTER ).exec()
            .then( async (doc) => {
                const match = bcrypt.compareSync( password, doc.password );
                console.log('then(): match? ' + match);
                    if ( match != true ) {
                        return callback(new Error('Invalid username or password.'));
                    }

                    // Since from this point on, we can assume that 'match' == true,
                    //  and that the user is authenticated.

                    if ( doc.enabled != true ) {
                        return callback(new Error('User access is disabled.'));
                    }

                    if ( doc.expires != null && doc.expires < Date.now() ) {
                        return callback(new Error('User access has expired.'));
                    }

                    doc.lastLogin = Date.now();
                    await doc.save();

                    console.log('authenticateUser(): ' + username + ' authenticated successfully.');

                    return callback(null, { username: username, relay: true });
        });

    } catch ( error ) {
        console.error(error);
    }
}