In addition to Emily's comment above I recommend adjusting your Retry Intervals under Settings>General>Spool to:
1, 1, 5, 5, 15, 30, 30, 30, 30, 60, 90, 120, 240, 480, 960, 1440, 2880
Please also change the Notify Senders of Delay After X Attempts to 7. If this does not resolve these issues I recommend submitting a support ticket so we can take a closer look at this.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com