How to delay sending of all emails in Outlook

Step 1: On the File tab, click Manage Rules & Alerts. Or, on the Home tab, in the Move group, click Rules > Manage Rules & Alerts:

Step 2: In the Rules and Alerts dialog window, click New Rule.

Step 3: Under Start from a Blank Rule, click the Apply rule on messages I send option, and then click Next.

Step 4: If you want to delay emails that meet certain conditions, select the corresponding check box(es).
For example, to delay messages sent through a specific account, check the “though the specified account” box, and then click Next.

To delay sending of all emails, do not check any options, simply click Next.
Outlook will ask you to confirm that you want the rule to be applied to every message that you send, and you click Yes.

Step 5: In the upper pane, under Step 1: Select actions, check the defer delivery by a number of minutes box.

Step 6: In the lower pane, under Step 2: Edit the rule description, click a number of link.
This will open a small Deferred Delivery dialog box, where you type the number of minutes for which you want to delay delivery
(a maximum of 120), and then click OK.

Step 7: The link now displays the time interval for which Outlook will delay sending of emails.
At this point, you can already click Finish to save time. Or you can click Next to configure some exceptions and/or
give an appropriate name to the rule. To walk you through the whole process, we click Next.

Depending on whether you want any exceptions or not, select one or more check boxes or click Next without selected anything.

Step 8: In the final step, give the rule some meaningful name, say “Delay sending email”,
make sure the Turn on this rule option is selected, and click Finish.

Click OK twice – in the confirmation message and in the Rules and Alerts dialog box.

Credit : https://www.ablebits.com/office-addins-blog/schedule-email-delay-delivery-outlook/