Undeliverable email messages via Office 365 Exchange

[Robert Keith]

Hi,

We have been getting messages that mail delivery has failed. Evidently the error code is 4.4.7. They look like this:

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Delivery has failed to these recipients or groups:

tools2242@gmail.com
Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.

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We have not sent any emails to tools2242@gmail.com. I have no idea what that address is. The emails are getting through to their intended recipients and then, a few days later, we get these messages. Not all the emails we send result in this message but we get many of these every day and it is becoming a nuisance.

I have checked all possible connections with our CRM, Google Calendar etc. and none of them could be doing this.

Why is Office 365 Exchange trying to send our mails to this gmail address?

Thanks in advance,

Rob

[#2]

This is most likely a case of "backscatter" as detailed here: https://blogs.msdn.microsoft.com/tzink/2014/11/22/improving-backscatter-detection-with-boomerang/

[#3]

I'm having the same issue as well. Thanks for raising it up

[#4]

I thought of that. However, all NDR messages are for emails we have actually sent to real recipients, and which have got through. Somehow, Office 365 Exchange is trying to send our legitimate emails to a non-existent address.

Thanks for the suggestion of blocking all NDR messages. It will certainly lower the rate of complaints I am getting about them.

A further mystery is that both affected people use different mail clients. One uses Outlook and I use a Web based service to send and receive.

In any case we are all moving to Zoho mail soon.

Rob

[#5]

Double-check that your SPF is tight. Publish DKIM and DMARC records if you haven't yet.

[#6]

What does message trace in EAC say? If that still doesn't show any clues, it may be time to open a support ticket with Microsoft. Use 'Start-HistoricalSearch' with -MessageTraceDetail parameter to generate a full MTL for them.

[#7]

@Robert Keith How is  ZOHO mail going for you?