Edit: This doesn’t work anymore I don’t think, because more modern versions check for SMTP conflicts and don’t allow this.

When you’re installing zimbra in an Ubuntu or Debian machine, it seems it installs the MTA in such a way that command line tools like mail and such don’t work. But when you install exim, it conflicts with the postfix in Zimbra.

To fix it, you can install exim4, but configure this line in /etc/default/exim4: