I just finished setting up postfix on a Debian 6 machine. All is peachy. However, when I checked my mail logs to see how things were going I noticed a number of entries for messages being sent from firstname.lastname@example.org to root. Checking root's mail led to messages with the following content:
/usr/share/sendmail/sendmail: line 880: /usr/sbin/sendmail-msp: No such file or directory
After some digging around, I found out that smmsp is an acronym for Sendmail Message Submission Program which is curious as I had removed sendmail during postfix's installation.
Or so I thought.
dpkg -l | grep send, I found that sendmail was still well and truly installed and was trying to send test e-mails to root using smmsp via a cron job. I proceeded to remove these vestigial packages using:
aptitude remove sendmail-bin sendmail-cf sendmail-doc
... and all was well. So far.
In eidetic hindsight, I also remember that upon installing and configuring postfix, it was unable to bind to the interface correctly. I'm guessing that this was because sendmail was still holding on in the vain hope that I would retain it. No thanks. Die vile demon! Die!
Hope this helps.