How to fix 550 550 Administrative prohibition - envelope blocked - [Moderator note: personal info removed]

Question

How to fix 550 550 Administrative prohibition - envelope blocked - [Moderator note: personal info removed]

Baguette_crusher 0

Hi all,

I've got a user on my tenant who has an AD (on-premise) account with an email address being : ******@mydomain.com with an smtp pointing to an external address from another tenant , let's say : ******@external.com. He is a mailUser on exchange (doesn't have a mailbox on our tenant, no e3 license, just mail-enabled : his email address is resolved and reaches to his external mailbox whenever someone sends his an email using ******@mydomain.com). Since yesterday, he was communicating with a third-party company (from a whole other tenant then) and the company in question was using the mydomain address to communicate with him. The problem now is that the emails from this company are not getting delivered. I've created a trace to track the emails and these are the message events :

User's image

The failure reason displayed is: Reason: [{LED=550 Administrative prohibition - envelope blocked - [Moderator note: personal info removed]

We do have mimecast in our system as an external layer (it's configured as : user | o365 | mimecast | outside) and mimecast says that this sender is blocked at policy level. The other users from our tenant are communicating fine with this external company and there is no policy preventing them from sending emails to the users in our tenant.

My theory is that this comes from the external tenant in which he has his mailbox.

I'm a bit confused, any thoughts on that?

Many Thanks!

0 comments

1 answer

Your answer

Answer 1

Dear @Baguette_crusher,

Good day! Welcome to Microsoft Q&A forum!

Based on your description, I understand that emails sent from a specific external company to a mail-enabled user in your tenant are failing with a "550 Administrative prohibition - envelope blocked" error from Mimecast. The affected user does not have a local mailbox and is configured as a MailUser that forwards mail to an external mailbox, while other users in your tenant can communicate with the same sender without issues. You're trying to determine whether the block is originating from Mimecast, your Microsoft 365 environment, or the external mailbox tenant.

Based on the information provided, I recommend raising a support ticket with Mimecast for further investigation.

The message is being rejected directly by a Mimecast SMTP endpoint, and the detailed policy evaluation responsible for the "550 Administrative prohibition - envelope blocked" response is only available within Mimecast logs, not Microsoft 365 message traces.

Therefore, Microsoft Support team will need to look further into this to be able to identify whether the rejection is being caused by a blocklist entry, impersonation protection, envelope validation, or another policy, and advise if an allow-list or policy adjustment is required.

When opening the case, please provide the full SMTP error, sender and recipient addresses, message trace details, the timestamp of the failed message (UTC), and the Mimecast server involved.

I appreciate the troubleshooting you've already completed, and engaging Microsoft Support team will be the most efficient way to determine the root cause and restore mail flow.

Please note that this is a community forum where users help one another. I do not have control over user accounts, including login issues, password resets, or access changes.

Here’s what your Microsoft 365 Global Admin should do next:

Submit a Service Request: The Global Admin should create a service request with Microsoft support to get advanced technical assistance and diagnostics. For detailed instructions on how to get support, please refer to this article: Get support - Microsoft 365 admin.

Thank you for your kindness and understanding!

If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in [our documentation] to enable e-mail notifications if you want to receive the related email notification for this thread.