OK, I don't know much about this, (I'm happy enough with Thunderbird, and mutt for when I'm forced to read system-generated messages about failed cron jobs, etc) but some things come to mind.
First of all, I believe that Debian (default) uses exim, rather than sendmail.
Second, how did you configure your mail when installing Debian? There's about 5 different standard options during the install, ranging from no mail handling at all to serving/hosting your own.
You can rerun the email confiuration routine with
# dpkg-reconfigure exim4-config
The debian install manual has a section that covers this.
8.5. Setting Up Your System To Use E-Mail
Hope this helps.