Ask HN: How do you stop an app's email getting flagged as spam?

Just saw Reddit's state of the servers report which included information on how sending email via Amazon EC2 causes it to get flagged as spam thanks to Trend Micro.

Regardless of whether you are using EC2 or not - how do you try to stop your app's email getting flagged as spam?

What can you do to run tests on your existing email to make sure it is getting through?