Ubuntu – DynDNS and mail servers

I have set up an Ubuntu box with EXIM4 for handling my email, for now I only care about outgoing email. My router manages my DynDNS.com psuedo static-IP addressing. Port forwarding for port 25 is open and shows up with a port-scan. The firestarter firewall on the box shows access attempts and I have added a permit rule.

Outgoing email seems to work as I wanted, and it is configured to send through my gmail account. The problem is that GMail is spamming me every 10-20 minutes with a Delivery Status Notification message complaining about connection refused. It seems to be that Google is trying to email my root@<dyndns-account> account.

How should I overcome this problem? How could I set up complete in/out email access? What is the best course of action for a private web-site that needs minimal email support (e.g. the MediaWiki I'm hosting, etc).

EDIT – added an anonymous copy of the email

    This is an automatically generated Delivery Status Notification

    THIS IS A WARNING MESSAGE ONLY.

    YOU DO NOT NEED TO RESEND YOUR MESSAGE.

    Delivery to the following recipient has been delayed:

        root@mydomain.is-a-geek.com

    Message will be retried for 2 more day(s)

    Technical details of temporary failure:
    The recipient server did not accept our requests to connect. Learn more at http://mail.google.com/support/bin/answer.py?answer=7720
    [mydomain.is-a-geek.com (1): Connection refused]

      ----- Message header follows -----

    Received: by 10.210.89.7 with SMTP id m7mr1107333ebb.14.1246185600392;
           Sun, 28 Jun 2009 03:40:00 -0700 (PDT)
    Return-Path: <me@gmail.com>
    Received: from MyMachine (isp.com [xx.xx.xx.xxx])
           by mx.google.com with ESMTPS id 10sm7080189eyz.31.2009.06.28.03.39.59
           (version=TLSv1/SSLv3 cipher=RC4-MD5);
           Sun, 28 Jun 2009 03:39:59 -0700 (PDT)
    Received: from smmsp by MyMachine with local (Exim 4.69)
           (envelope-from <smmsp@mydomain.is-a-geek.com>)
           id 1MKrnl-00075G-Oe
           for root@mydomain.is-a-geek.com; Sun, 28 Jun 2009 11:40:01 +0100
    From: Cron Daemon <me@gmail.com>
    To: root@mydomain.is-a-geek.com
    Subject: Cron <smmsp@MyMachine> test -x /etc/init.d/sendmail && /usr/share/sendmail/sendmail cron-msp
    Content-Type: text/plain; charset=UTF-8
    X-Cron-Env: <MAILTO=root>
    X-Cron-Env: <SHELL=/bin/sh>
    X-Cron-Env: <HOME=/var/lib/sendmail>
    X-Cron-Env: <PATH=/usr/bin:/bin>
    X-Cron-Env: <LOGNAME=smmsp>
    Message-Id: <E1MKrnl-00075G-Oe@MyMachine>
    Date: Sun, 28 Jun 2009 11:40:01 +0100

      ----- Message body suppressed -----