Comment 8 for bug 42947

I think it likely has to do with a dependency issue. First, I've just done this on 6.06, but I think that if you remove Postfix entirely and then apt-get -V install it, you will see some packages 'not upgraded'. Figure out what those are and solve that problem and I suspect you will be OK.

sudo dpkg --remove postfix
sudo dpkg --purge postfix

Then reboot to be on the safe side (see a comment above), then

sudo apt-get -V install postfix

Pay particular attention to packages not installed or upgraded.