Postfix daemon missing?

Reinstall would be the quickest and easiest, depending on which distribution you're running. For Debian/Ubuntu,

apt-get --reinstall install postfix

I'm 85% sure this will not overwrite any existing configuration, but of course, you probably want to back everything up anyway (files don't normally delete themselves). Or, if you have a system with the exact same architecture, distribution, and release, you can simply copy that specific file to the disadvantaged system.

Thanks for the suggestion. What I couldn't figure out is that I'm pretty sure I had postfix running successfully a few weeks ago.

I did see the following interesting output on running the reinstall command listed above (have a look at the packages to be removed):

Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  procmail postfix-mysql postfix-pgsql postfix-ldap postfix-pcre sasl2-bin
  resolvconf postfix-cdb ufw
The following packages will be REMOVED:
  exim4-base exim4-config exim4-daemon-light
The following NEW packages will be installed:
  postfix
0 upgraded, 1 newly installed, 3 to remove and 0 not upgraded.
Need to get 0B/1321kB of archives.

Scratching my head trying to figure out how exim is related here? I wonder if that's what messed up the install of postfix. I'll continue to dig a little more and post what I find.

I first set up exim to receive mail from this guide, and then later added the ability to receive mail with postfix using this guide guide.. I'm wondering if perhaps something got messed up along the way in that these two things aren't meant to be used simultaneously..?

Although I did stop at the postfix configuration step for sending emails, here.

You can only have one MTA installed at a time. When exim was installed, postfix was uninstalled (and installing postfix would uninstall exim). Good news is that they both do the same things, more or less. So, installing postfix per that guide will obviate the need for exim, since it will be able to send mail too.

Postfix and Exim both provide and conflict with the mail-transport-agent package which means only one can be installed at a time.

Excellent– thanks for the help!! I'll pick one of the two and finish out my configuration.

Got everything fixed.. thanks again! I'm sure what was mentioned above is what happened– probably when I went to install exim I didn't see the warning that postfix was going to be uninstalled (or at least part of it).