Mark - I think you have to set up a rule.
Let us know if this works:
For Outlook:
Go to: "Tools"/ "Rules & Alerts"
Select "New Rule" and "Start from a blank rule".
Select "Check message after sending".
Select "On this machine only".
Then: "Defer delivery by #(=a number of minutes)".
Insert the number of minutes delay required.
"OK" and "Finish".