Weird behavior of Exchange Online for none-existing email address

Thanks Arindam_Thokder and VasilMichev . With your help, I was able to verify EOP's behavior and find a solution.

The blog link send by Arindam provided the information I need.

I have confirmed EOP will behave this way:

1. if the sender IP is not included in any tenant's incoming connector, the message is considered as Incoming, and "RCPT TO" will work as expected for any receipt email address.

2. If the sender IP is include in one or more tenant's incoming connector, the message is considered as Originating and things are getting more interesting.

For originating message, if the "mail from" and "rcpt to" email address match one of the setup incoming connector, then "rcpt to" will work as expected. If either of the "mail from" or "rcpt to" doesn't match the tenant's email, "rcpt to" will always return 250.

So to fix the problem, I will need to create a incoming connector in my tenant with the IP from Fortimail, then change Fortimail 's "Recipient address verification" from "use system setting" to "use domain setting".

So
mail from:noreply @ my email domain .com 
rcpt to:my email domain .com  

will work as expected ( return 550).

mail from: noreply @ Fortinet .com 
rcpt to:my email domain .com 

won't work.