Plugins are usually installed in the same way, unless specified otherwise in their documentation.
For the time being, plugins are all shipped with the
thus there is no need to download them separetly.
Link to configuration
Now that the desiered plugin is installed on the system, it is usually recommanded to
link the library to the vSMTP configuration for a specific service with the
ln -s /usr/lib/vsmtp/<vsmtp-version>/<plugin> /etc/vsmtp/<service>/plugins/<plugin>
ln -s /usr/lib/vsmtp/3.0.0/libvsmtp_plugin_mysql.so /etc/vsmtp/working/plugins/libvsmtp_plugin_mysql.so