365 Postmaster Spam

Hello Experts,

Currently we have all of our Exchange accounts in 365.
I set myself to get the "postmaster" e-mails on undeliverable.

I am currently getting a lot of these?

Subject: Undeliverable: Your Flight Ticket Invoice 53518 - (our user)
Sender: ourcompany@onmicrosoft.com

Our we really sending these out?  I am concerned and not sure how or why our spam is increasing.

I found this question on the website and it's what is going on with us.  We have a spn record on our domain and they are coming frequent, so as of typing this, I"m unclear if there is anything or any other settings I can do to protect our domain?
https://www.experts-exchange.com/questions/28946520/Spoofing-email.html

Thank you for your help,

Karen
klsphotosAsked:
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Vasil Michev (MVP)Commented:
This is known as "NDR spam" or backscatter, and is a commonly used technique by spammers. There's actually an option in EOP you can configure to increase the spam score for such messages, and hopefully get them filtered/marked as junk before they hit your mailbox. Read here: https://blogs.msdn.microsoft.com/tzink/2014/11/22/improving-backscatter-detection-with-boomerang/
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klsphotosAuthor Commented:
Thank you so much, so far so good!
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