Hello,

I have a few users using office 365 pro plus E3 in a Hybrid environment, some mailboxes are in the cloud and some are on premise. They are stating that some email with mostly Excel attachments are arriving to the recipients without the attachments.

They do not receive NDR message or any indication that exchange, Firewall or Antivirus has stripped the attachments. This does not happen all the time, While troubleshooting I asked a user to forward the known removed attachments to me and I received and was able to open them without issue. I was able to open in Outlook on my Laptop, on my Mobile Phone, and in OWA.

It is a strange issue that I was hoping someone may have experience before and could provide some insight.

Best Regards,

Neil Jones

1) First i would recommend you to validate the email with attachment from "sent items" of the sender. Confirm it contains the attachment or not.
2) You can download latest MFCMAPI utility (mfcmapi.codeplex.com) and open the targeted user profile and open the mailitem. Validate that the attachment exists (most probably it would be there as you can see its OWA).
3) Check with Exchange On-Prem Admin if there is any GPO or Exchange policies which are blocking it.
4) You can enable Pipelin tracing at On-prem Exchange and validate it.

In addition, you can check the KB https://support.microsoft.com/en-us/kb/287720 and related post: https://social.technet.microsoft.com/Forums/exchange/en-US/5ddd2f7f-9613-4744-ada5-9893d58cd18b/outlook-not-showing-attachment-owa-does