When making changes to the configuration file, it is often helpful to make sure that your changes are valid. The easiest way to do this is to do an informative syntax check:
The -t option directs the server to parse the configuration file but stop before actually starting its operations as a server. The -d5 will cause the server to display debugging messages during this testing of the configuration file. Another useful command is:
proftpd -c /path/to/new/config/file -td5
which lets you test the syntax of some new configuration file before it is put into production.