Emails to Microsoft 365 customers are silently dropped

Question

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

Answer 1

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

Answer 2

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

Answer 3

Hi @Michael Jansen ,
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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