SMTP not working on shared mailbox on specific tenant..

Question

I administer multiple tenants.

I've set up countless send via SMTP from shared mailboxes in M365, but now I have one that doesn't work. This is the errorr displayed when sending via Powershell:

Send-MailMessage: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful, the request did not meet the criteria to be authenticated successfully. Contact your administrator. [VI1PR07CA0135.eurprd07.prod.outlook.com 2025-06-16T11:04:36.851Z 08DDAA1549C652B6]

Enabled SMTP on tenant and client...I've disabled security defaults on tenant, since I once had a problem, if that is enabled. I changed password, since that caused problems also once. I created a new account it there was maybe a problem with the account... the problem remains.

Any suggestions what can be a problem?

Answer 1

Dear Miha,

My sincere apologies for the delayed response to your ongoing issue with SMTP sending from your shared mailbox. I understand this problem has been quite persistent and challenging to resolve, especially given the extensive troubleshooting you've already performed.

Given the complexities involved, particularly with the observed differences between your older and newer tenants, and the specific authentication error you're receiving, this type of issue often requires a deeper backend investigation into your specific tenant's configuration and policies.

Therefore, the most effective next step is to submit a support ticket directly to Microsoft 365 Support. They have the necessary tools and access to diagnose server-side authentication failures and tenant-specific settings that are not exposed through standard administrative consoles. 

How to Submit a Support Ticket to Microsoft 365 Support:

1. Go to the Microsoft 365 admin center.
2. In the navigation pane, select Show all > Support > New service request.
3. Follow the wizard to describe your issue in detail, including all the troubleshooting steps you've already taken.

Once you have created the support ticket, please share the Support Ticket ID with me via a private message here in the Microsoft Community. Providing me with this ID will allow me to escalate your case to our technical team for further review and faster resolution.

To view private email options, sign in to Microsoft Community> use the email account that posted this thread information to Microsoft Community> > click Profile in the upper-right corner, and then select My Profile.  

I am fully committed to helping you find a resolution, and taking this step will allow your issue to be placed directly in front of the engineers who can perform the necessary deep dive and relsove your issue.

Kind regards,

Kai-L - MSFT | Microsoft Community Support Specialist

Answer 2

First, let me clarify again! This is a shared mailbox with no licence attached. From few of your comments I figured youre talking about a licenced mailbox.

Checked!

Get-CASMailbox -Identity "UserAlias" | Format-List SmtpClientAuthenticationDisabled
SmtpClientAuthenticationDisabled : False

This one, I can't check, since mailbox is shared and unlicenced.

Check via Microsoft 365 Admin Center:

Go to Users > Active Users.
Select the user account you're using for authentication.
In the flyout pane, go to "Mail" > "Manage email apps".
Ensure "Authenticated SMTP" is checked.

I don't have Send as or Send on Behalf permisions set.

I don't have licence that would allow configuring conditional access.

I did audit search for that particulart mailbox but no results came back. Maybe i defined my search wrong? (I defined time frame, all activties with sent and for that specific user.

I now tested one of my last tenants I've set up and it's not working there either, but working on older tenants that I administrer.

Did Microsoft change something about sending SMTP from shared mailboxes?

Answer 3

Dear Miha,

Thank you for posting in Microsoft Community.

I understand you're experiencing difficulties with sending emails via SMTP from a shared mailbox in Microsoft 365, specifically encountering the error "5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful." This error typically indicates an authentication failure related to how SMTP AUTH is configured or utilized with shared mailboxes.

Based on the information you've provided and common troubleshooting practices, let's go through the potential causes and solutions:

1. Re-confirm SMTP AUTH is enabled for the Mailbox/User:

Even though you mentioned enabling SMTP on the tenant, it's crucial to ensure it's specifically enabled for the user account you are using to authenticate when sending from the shared mailbox.

Check via PowerShell:

Get-Mailbox -Identity "UserAlias" | Select-Object SmtpClientAuthenticationDisabled

If this returns True, SMTP AUTH is disabled for that user. You need to set it to False:

Set-Mailbox -Identity "UserAlias" -SmtpClientAuthenticationDisabled $false

Replace "UserAlias" with the actual user account you're using to authenticate (not the shared mailbox itself, but the user who has "Send As" or "Send on Behalf" permissions).

Check via Microsoft 365 Admin Center:

Go to Users > Active Users.
Select the user account you're using for authentication.
In the flyout pane, go to "Mail" > "Manage email apps".
Ensure "Authenticated SMTP" is checked.

2. Verify "Send As" or "Send on Behalf" Permissions are Correct:

While the error is about authentication, sometimes misconfigured permissions can lead to unexpected behavior.

Send As: This is generally preferred for shared mailboxes. Ensure the user account you're authenticating with has "Send As" permission on the shared mailbox.
PowerShell:

Add-RecipientPermission -Identity "SharedMailboxName" -AccessRights SendAs -Trustee "UserAlias"

Send on Behalf: If you intend to send on behalf, ensure those permissions are set up correctly.
PowerShell:

Set-Mailbox -Identity "SharedMailboxName" -GrantSendOnBehalfTo "UserAlias"

3. Check for Conditional Access Policies (More Deep Dive):

Even though security defaults are off, check your Azure AD Conditional Access policies. Look for policies that might:

• Block legacy authentication (which SMTP AUTH often falls under).
• Require MFA for all cloud apps, even if the user isn't directly prompted, it could be failing silently.
• Target "Exchange Online" or "All Cloud Apps" and have conditions that are being met (e.g., device state, location).

4.Review Audit Logs in M365 Security & Compliance Center:

This can provide more detailed information about why the authentication failed.

• Go to the Microsoft 365 Security & Compliance Center.
• Under "Search" > "Audit log search".
• Search for activities related to "User logged in" or "Mailbox activities" around the time of the failed send attempts. Look for failed login attempts or authentication errors related to the user account you're using.

5. Open a Microsoft Support Ticket:

If none of the above steps resolve the issue, I recommend contacting Microsoft Support for deeper assistance:

Go to the Microsoft 365 Admin Center, go to Support > New Service Request
https://admin.microsoft.com/#/support/requests

As Microsoft Community moderators, we don’t have access to internal logs or backend systems due to privacy and security policies. For this reason, working with official support ensures better troubleshooting and escalation if needed.

For reference: Get support - Microsoft 365 admin | Microsoft Learn

If none of the above solutions work, let me know for further investigation. Please note that our initial response does not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution.
Kind regards,

Kai-L - MSFT | Microsoft Community Support Specialist