Outgoing Emails Fail

I'm trying to send out a newsletter via Simplenews, whenever I try to send it out via Mimemail the following error shows up in my log and the email is never delivered:

Type simplenews
Date Monday, May 25, 2009 - 11:47pm
User Nick Lapp
Location http://mysite.com/admin/reports/status/run-cron
Referrer http://mysite.com/admin/reports/status
Message Outgoing email failed. Message type: node
Subject: [Newsletter] Newsletter
Recipient: drupal@mysite.com Sent via Mime Mail
Severity error

Unfortunately it doesn't tell you *why* it failed. Any thoughts? The newsletter is a big part of this website.