The latest builds of Office 2016 for Mac include a frequently requested feature: Read & Delivery Receipts. However, it is only supported in Office 365 accounts (business and enterprise only). Look for it in the Insider Fast build (15.34.17404) and general availability in spring 2017. The option to request read and/or delivery receipts is on the compose mail Options ribbon and Draft menu.
If you are not familiar with receipts (because Outlook for Mac did not support it), receipts are notification that the message was received or read.
- Delivery receipt: send a notification mail when the email has been delivered to the mailbox.
- Read receipt: send a notification mail when email has been opened by the recipient
Not all email clients can return read receipts and recipients usually have the option to not return one. Neither receipt option will guarantee you that the message was placed in the Inbox or read. The read receipt only tells you that the recipient marked the message read. If the recipient reads it using a client that does not support receipts or has an option to not return receipts, you won't get one back. A 'deleted unread' receipt tells you that the recipient deleted a copy of the message without marking it Read, it does not guarantee they didn't read it without marking it read.
Delivery receipts, if supported, will tell you the message made it to the recipient's mailbox.
If the server doesn't support returning receipts, Office 365 generates a message that the message was handed off to the server and Office 365 doesn't know what happened to it.
If the recipient uses an email client that supports responding to read receipts, you should receive notification when the message is read or deleted unread, provided it's not filtered out by antispam software.