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

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,