reddit's email system lives primarily in two places, a lib and a model, although as an end-user you'll probably only need the library.
The entry-point for emails is emailer.py's _system_email(). However, you'll generally want to wrap this in a little bit of logic specific to your particular email use-case. This is what verify_email() does, for instance, for email-verification emails.
_system_email() chucks your carefully-crafted email into a queue to be processed later. This allows fast web responses back to the user, batching, throttling, and some other cool stuff, but makes life a bit more difficult in development. There's [[an email cronjob|Cron-jobs#email]] you'll need to kick off manually when you want to process queued emails:
[$]> sudo start reddit-job-email
If you have level = DEBUG in the [logger_reddit] section of your .ini file, you should see output from the job in /var/log/upstart/reddit-job-email.log.
Oh! One more thing: if you don't have an SMTP server configured in your development environment (that'd be a bad way to accidentally send out a bunch of emails), you'll need to launch send_queued_mail() with test=True. /etc/init/reddit-job-email.conf is a copy of the upstart file from the repo; you need to just edit it and pass in True to the function. That will bypass trying to send any emails and print them out to the log file instead.
In summary:
Setup: write code to call _system_email(), modify reddit-job-email.conf.
Testing: start reddit-job-email, check reddit-job-email.log.