Correct location for ActionMailer interceptor

I read the Rails documentation for creating an email interceptor. It
mentions putting the registration in config/initializers, However,
no mention about where I should store the interceptor itself. What is
best practice for this?

To get started:

For Rails 3,

Railscasts: #206 Action Mailer in Rails 3

At Github, Rails ActionMailer, the interceptor test file is pretty
interesting, around line 642

rails/actionmailer/test/base_test.rb at 3e36db4406beea32772b1db1e9a16cc1e8aea14c · rails/rails · GitHub.

Thank you Elizabeth. I’m new to Rails so poking around in the Rails
is insightful.

Usually the test case is a good guide for how to use a piece of code.
interceptor is declared right inside the test case. The documentation
the test seem to imply that I should declare the interceptor and the
registration in the same file and place it inside the initializers

I’m implementing an email interceptor that redirects outbound email to a
testing email account during development, so I added an interceptors
under config and put the registration in development.rb. This seems a
sensible place for interceptors to live. It also registers the
only under the right circumstance.

Feedback about this is appreciated,