Recently I noticed that Postfix had stopped logging information about emails which I sent through my VM (I run mutt locally over SSH in a screen session). On closer inspection, it turned out that most of the files in /var/log had also stopped updating since October, which is when I upgraded my VM from Squeeze to Wheezy. After a bit of searching, I discovered that the syslog daemon was removed as part of the upgrade, even though it is available for Wheezy. Reinstalling the package using the following command did the trick:
apt-get install inetutils-syslogd
All the logging is now back to normal, and I can check whether emails have been sent or delayed for some reason (e.g. greylisting).