Teams email invitations not sent when the participant is in the same domain

[Mark_Sfn]

Hello,

When we're setting up a meeting, if we add a colleague who shares the same email domain with us, we noticed that the email invitation is never sent to that user. However we are able to send email invitations to users with different email domains. 

So all in all, suppose I have an email address like mark [at] example.com and I create a Teams meeting with joe [at] another-example.com and john [at] example.com, the first participant will get the email while the second (which is in the same domain as me) won't.

Using Exchange Admin's Mail flow, this is the output I get for the message that wasn't sent to my colleague.

Received -> Not Processed -> Not delivered

Status: Unfortunately, we aren't able to provide an analysis for this message at this time.

More information: You can review the Message Events table below for details about the status of this message.

Date (UTC+02:00) | Event | Detail |
------------------------------------
6/30/2022, 10:23 PM | Redirect | No detail information available.

More information
Message ID:redacted
Message size | From IP | To IP
‎34.24‎ KB | 52.114.xx.xxx | null

Any ideas on how to resolve this?

Thank you in advance.

[#2]

I'm going to guess that there is some configuration in your mail flow that is blocking it. The simple fact that it shows received definitely leads to that conclusion.

[#3]

I agree with you but you have a look at the IP addresses a bit below, the destination IP is null, which makes me think that for some reason MS cannot resolve the domain.