Request a delivery receipt in Outlook
If you want to be especially sure that an email message has made it to a specific target, you can always request a delivery receipt.
To do so in Outlook, just open a new message, then head to the “options” tab. There, at the top of the page, click on “request a delivery receipt”.
Note that this is different from a read receipt, which will tell you when an email has been read, not just delivered.