Sometimes, when sending an email in a Soapbox Mailer Campaign, you'll find that the email isn't sent, and when you go to Setup > Monitor > Jobs > Apex Jobs, you'll see the following in the Status Detail column: "First error: Attempt to de-reference a null object". Here's how to troubleshoot this issue.
1) Ensure that you have the correct Amazon Settings
a) Go to the Amazon Settings tab
b) Re-enter your Access Key ID and Secret Access Key (note: even though you may have entered this data in the past, it will be blank now)
c) Click the Save button
d) Try resending your email
2) Ensure that you have added content to the HTML version of your email
3) Ensure that you have added content to the text version of your email