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MichaelJansen-7812 avatar image
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MichaelJansen-7812 asked LucasLiu-MSFT commented

Emails to Microsoft 365 customers are silently dropped

Hi!

We run a SaaS customer communication tool called Flixcheck. When sending mails to our customers, we are using an email service provider (CM.com).

Some of our customers (namely those who use Microsoft 365 / Exchange), do not get our emails.

We already investigated this issue with our email service provider, but according to their logs, all emails are perfectly accepted by the M365 servers and not bounced.

Still, our customers say that the emails don't shop up in their inboxes and also do not show up in their junk boxes. Obviously, our emails are silently dropped by M365.

How do we find out, why our mails are dropped and how to stop it? Our customers say that they already whitelisted us as sender, so they can't help here either.

Best regards
Michael

office-exchange-server-mailflow
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AlexC-8264 avatar image
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AlexC-8264 answered

Hi Michael,
Are you / your service provider sending those mails in the name of someone else, even the recipient domain itself?
Do you have contact with the tenant admin(s) of the recipients?
Let them check in their M365 quarantine, if those e-mails got filtered by Exchange Online Protection because of SPF=fail or DMARC=fail.
With appropriate permission they can access quarantine through https://protection.office.com/quarantine .
May also be, that the Bulk mail or Spam mail options are tuned a bit too tight... ;o)

KR, Alex

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LucasLiu-MSFT avatar image
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LucasLiu-MSFT answered LucasLiu-MSFT commented

Hi @MichaelJansen-7812 ,
Agree with above.
Do customers who using Exchange/Microsoft 365 all have this issue?
Do they check the email in OWA?

1.If possible, please check if there have related tranpsort rule in your customers organizatoin.

2.Please check the message trace in Exchange admin center to view if there have any related mail records.

3.Please try to following the steps in the link provided by Andy to create safe sender lists.



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Hi Lucas,
Thank you for your response.


Do customers who using Exchange/Microsoft 365 all have this issue?

No, not all, only several.


Do they check the email in OWA?

Most of them use M Outlook.


1.If possible, please check if there have related tranpsort rule in your customers organizatoin.
2.Please check the message trace in Exchange admin center to view if there have any related mail records.

We will check for that.

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Hi @MichaelJansen-7812 ,
I am writing here to confirm with you how thing going now?

1.Are these users in the same Office 365 tenant?
2.Checking OWA is to prevent the issue from being caused by Outlook.
3.You could also check the Quarantine in Office 365 Security & Compliance.
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Hi Lucas, thank your for your response.

  1. No, they are completely independent.

  2. They also do not appear in the OWA

  3. We found several of the lost mail in the quarantine! We already checked it after this answer: https://docs.microsoft.com/answers/answers/357170/view.html

I need to wait for my colleagues to confirm that all customers found their mails. As soon as that happens, I will click "Accept Answer". Thank you!

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AndyDavid avatar image
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AndyDavid answered AndyDavid edited

The customers will need to open a ticket with 365 support.

P.S.

You could always point them to this doc as well to how to correctly safelist messages:

https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/create-safe-sender-lists-in-office-365?view=o365-worldwide

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First of all, thank you for the quick response! I've already searched for it but was unable to find a form or ticketing system where I can point our customers to. Could you provide me the correct link to the 365 support?

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Yea, they can open on the portal.

https://docs.microsoft.com/en-us/microsoft-365/admin/contact-support-for-business-products?view=o365-worldwide


I say that because if they have truly whitelisted your email, then they should be delivered.
So either they didnt whitelist correctly or somethin else is going on.

P.S.

You could always point them to this doc as well to how to correctly safelist messages:

https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/create-safe-sender-lists-in-office-365?view=o365-worldwide

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