You likely have an email stuck in the Outbox.
Outlook is nowhere near the horror show it used to be but it is a complicated beast that wouldn't know an understandable error message if it bit it.
So look down the left hand side of your Outlook window in the folders. There is likely one there called Outbox and it is in bold possibly with a "(1)" next to it.
If you want to search in Finder for the same, and then do the equivalent in Mail, you should go to /Users/you/Library/Mail/V* * varies for MacOS. Do either of these 2:
In the question, you mentioned that you can search for specific messages inside mail app. So just pick a rare keyword from one of them and search for the same string in the directory. Press ...
I hope someone comes up with something better than this, but…
X-out the search - this awkwardly leaves you in the now empty search box rather than in the mail list.
Select any mail in the list. [Or hit Tab until you get back into the list. Depending on how you have your tabbing set up that could be three presses, or about 10:\ ]
Press Home [or Fn/Left Arrow ...
So, check the following in Gmail:
Open up Gmail
Click the gear in the top right to open settings
Ensure the 'Show in IMAP' checkbox is ticked next to any labels you wish to see as folders in Apple Mail.
I hope this answers your question, and that this is what you where looking for.
There are two header tags related to receipts:
Return-Receipt-To This is asking for a verification that the mail got to your inbox. I was surprised that mail responded by sending the original message back to me. macOS 10.10.5 I see this as a minor privacy issue in most circumstances. Could be used to tell if your work computer was powered on and mail app ...