Remote Server returned '550 5.7.1 Message rejected due to unacceptable atta

[franklinf]

Hi,

Currently I have a hybrid implementation Exchange 2013 and Office 365. In the local datacenter I manage two 2016 EDGE Exchange servers located in the DMZ.  The EDGE Servers is only for mail flow between O365 and On-premise and viceversa.  I have a mail gateway on the edge to filter all incoming messages.

However, a couple of days ago a person external to the company sent a Webex invitation from Outlook through an AddOn called Oracle Beehive Extensions for Outlook. For non-migrated mailboxes, the invitation arrives correctly. But for a mailbox migrated to O365 the invitation does not arrive and the mailbox servers return the following message:

Delivery has failed to these recipients or groups:

usercloud01@contoso.mail.onmicrosoft.com

Your message was not due to a permission or security issue. It may have been rejected by a moderator, the address may only accept email from certain senders, or another restriction may be preventing delivery.

The following organization rejected your message: mail.contoso.com.

Diagnostic information for administrators:

Generating server: MAILBOXEXC003.contoso.local

usercloud01@contoso.onmicrosoft.com
mail.contoso.com
Remote Server returned '550 5.7.1 Message rejected due to unacceptable attachments'

I was reviewing this information to validate if it was only to remove an extension so that the filter in the EDGE Transport service would omit it. However, apparently it is filtering by content type and not by extension.

Workarround executed:
1.-  I applied this solution without positive result: https://support.microsoft.com/en-sg/help/945046/you-receive-an-ndr-message-when-you-send-an-e-mail-message-by-using-an  
2.-  I configured the domain as BypassSenderDomains adding the webex.com domains, etc without positive result. Ref: http://woshub.com/manage-domains-and-email-address-whitelist-in-exchange-server-2013/

The only way that the user "usercloud01@contoso.com" received the invitation was to disable the transport filter completely with the following command: Disable-TransportAgent "Attachment Filtering Agent" Ref: https://docs.microsoft.com/en-us/exchange/antispam-and-antimalware/antispam-protection/attachment-filtering-procedures?view=exchserver-2019#use-the-exchange-management-shell-to-enable-or-disable-attachment-filtering 

Attach document word with images and the error header received by the sender of the invitation.

DESIRED OBJECTIVE:
What I want is to be able to keep the EDGE side filter enabled so that it can allow the delivery of that webex invitation.

Any ideas or comments that can help me solve the problem?

Thanks by your help!

[terrylee]

Hello, jgough1, I think I can help you. The first thing I would do is remove the machine from the domain and connect to it again.

Log in to the administrator's account and go to Start, hold on to the right-click the Computer and choose Properties. Choose Change settings. Go to the Computer Name menu and choose Change.

Choose Workgroup, name it, and confirm it by OK. You are prompted to restart your machine now, do it.

Now go to the Computer Name and one more time choose Change.

Go to Domain, and name your domain. Confirm your actions by OK and log in with the account that has administrative rights in the domain. Restart, when you are asked to do it.

The next step is to read the step-by-step guide provided by Microsoft.

And the last piece of advice from me is to use NAKIVO backup software, which helps you recover from backups at any moment.