Thanks Jim, that solved it!
For completeness, I did run the install again. Here's how it went:
[root@myserver RT-Extension-CommandByMail-2.01]# perl Makefile.PL
Using RT configuration from /opt/rt4/lib/RT.pm:
etc=> /opt/rt4/local/plugins/RT-Extension-CommandByMail/etc
lib=>
I need some advice with getting the CommandByMail extension working. I've
followed the installation instructions here:
http://search.cpan.org/dist/RT-Extension-CommandByMail/lib/RT/Extension/CommandByMail.pm
I'm on 4.4 so I've commented out the old @Mailplugin lines in
RT_Siteconfig.pm
Here's