Using New Outlook on Windows for personal email, calendar, and contact management
I think I might have found a way to do this but I am not sure what version of Outlook I am on as it baffles me if Microsoft don't know this themselves....
You can set up a mailbox rule for all messages you send.
Home > Rules > Manage Rules & Alerts > New Rule
Choose "Apply rule on messages I send" and press Next, then next again (i.e. don't select a condition).
Outlook will flag that this rule will apply to All messages you send and ask if you want to continue.
When at the next step "What do you want to do with the message?", you can choose to delay delivery by a set number of minutes.
UPDATED to include image of Rules Wizard.