Welcome to the CapistranoMailer wiki!
If you install CapistranoMailer in your project and set it up before you have ever deployed the application you WILL get an error the first time.
This is because CapistranoMailer accesses all of the capistrano variables that are available, but when there are no previous releases, the attempt to access the previous release variables fails.
In other words, CapistranoMailer does NOT work on a ‘cold’, or ‘initial’ deploy. On subsequent deploys there is no problem, because there are always previous releases.