Comment 2 for bug 334410

sudo creates a mail if there is some problem with the authentication, which explains the initial error mesage. Perhaps more things in startup use sendmail, and thus they all fail because of this?

I guess it's questionable in the first place why you installed postfix on a desktop box if you don't use/configure it, but I guess it was installed as a dependency of something.

I'm not sure whether it would make sense to fix postfix to gracefully fall back if main.cf doesn't exist. I guess it wouldn't; my advice for you is to boot rescue mode, apt-get purge postfix (if you don't need it), and restart again.