This a solution, not a problem. Just wanted to share so it's out there for
anyone else searching.
Started getting warning: mysql:/etc/postfix/mysql_virtual_alias_maps.cf is
unavailable. unsupported dictionary type: mysql
apt said that postfix-mysql was installed. The mysql libraries were
present in /lib/postfix/.
But postconf -m failed to include mysql in the list of system modules.
Turns out, when trying to set up for some rsyncing of config files to my
secondary server, I added ACLs to the contents of /etc/postfix. Turns out
postfix does not like if any non-root user has access to dynamicmaps.cf.
It was still owned by root outright, but the additional access permissions
of the ACL still made it no bueno.
Once I removed the ACL and restarted, postfix and mysql played nice
together again.
