tested on redmine 4.0.4
to see the email previews put in config/additional_environment.rb
:
config.action_mailer.show_previews = true
config.action_mailer.preview_path = "#{Rails.root}/plugins/redmine_custom_email_template/test/mailers_previews"
and visit /rails/mailers/mailer/issue_add
or /rails/mailers/mailer/issue_edit