Very hard to troubleshoot and determine exactly what happened. As far as I can tell, I added a new email user for a virtual server and this (somehow) deleted the virtual aliases for 3 out of the four existing users. I then had to manually add them back to /etc/postfix/virtual and also 'resave' each of their accounts through Virtualmin.
There is the following error in miniserv.error:
/usr/sbin/postconf: warning: ?: unknown parameter