After installing OS X 10.8 Mountain Lion, a few things in our web development environment stopped working (namely Apache with multiple vhosts, BIND/wildcard DNS, Phusion Passenger and the rmagick2 gem, and postfix). The reason being is that Apple overwrites all of the configuration files that those tools rely on (although the old ones are all backed up with an appended ~previous, so fixing is relatively straightforward). We rely on postfix for sending mail locally via smtp using PHP’s mail() (i.e. sendmail) or Rails’ ActionMailer.
Fix for postfix in Mac OS X 10.8 Mountain Lion.
I didn’t find the backup copy of the plist – mine were just, simply, blasted.