Using ActionMailer with SMTP in a development environment

I just ran in to a problem where I wanted to use SMTP delivery for an ActionMailer mail in a Rails app running on my local development machine. When I sent the email, it seemed to send with no problems (no errors raised) but it didn’t get delivered and I couldn’t find it. So I changed my authentication credentials to supply a bad password, and still it didn’t complain. Ah, there’s the problem. It really should complain, with a

Net::SMTPAuthenticationError (535 Incorrect authentication data)

The Rails app needs to be restarted to pick up the configuration settings. This was on a Locomotive environment with Rails 1.2.3.