Why Is Exchange Online’s ‘Send As’ Feature Failing on Shared Mailboxes?

Troubleshooting Exchange Online Issues: Fixing ‘Send As’ Authorization Errors

Microsoft Exchange Online users have been facing disruptions recently, and it’s causing headaches for administrators and businesses alike. Over the past two days, issues have surfaced that directly impact productivity, particularly with the “Send As” feature for shared mailboxes. Let’s break this down, step by step, to understand what’s happening and how to address it.

What Happened?

On April 9, 2025, Microsoft Exchange Admin Center (EAC) experienced a global outage. Administrators attempting to access the EAC were met with an HTTP Error 500, rendering the service inaccessible. Microsoft acknowledged the issue under incident ID EX1051697 in the Service Health section of the M365 Admin Center and rolled out a fix later that day.

However, just as users were breathing a sigh of relief, another problem emerged on April 10, 2025: the “Send As” authorization for shared mailboxes stopped working. This means users who previously had permission to send emails from shared mailboxes can no longer do so.

What Does This Mean for Users?

When attempting to send an email using “Send As” permissions, affected users receive an error message stating:
“This message could not be sent. You do not have permission to send the message on behalf of the specified user.”

This issue has disrupted workflows for businesses that rely heavily on shared mailboxes for customer support, team communication, or other collaborative tasks.

Real-World Impact

Several users have reported this problem:

• Fermi, a blog reader from Germany, noted that his company’s support mailbox is affected.
• Christian R., another user, confirmed via email that his team is unable to use “Send As” permissions since this morning. He observed that emails bounce back with authorization errors.

These reports highlight a widespread problem affecting various organizations globally.

Possible Causes

While Microsoft has not provided an official explanation yet, there are several potential reasons for this disruption:

• Residual Effects from EAC Outage: The fix deployed to resolve the April 9 outage may have inadvertently caused new issues in Exchange Online’s permission settings.
• System Updates: Microsoft frequently updates its cloud services, and sometimes these updates introduce bugs or conflicts in existing features like “Send As.”
• Configuration Changes: It’s possible that changes were made to shared mailbox permissions or authentication protocols during maintenance.

What Can You Do?

If you’re experiencing this issue, here are steps you can take:

Step 1: Check Permissions

Verify that users still have “Send As” permissions assigned to their accounts via the Microsoft 365 Admin Center or PowerShell commands. Permissions may need to be re-applied manually if they were reset during recent updates.

PowerShell Example:

Get-MailboxPermission -Identity "SharedMailboxName"

Ensure the affected user has FullAccess and SendAs rights assigned.

Step 2: Test Alternate Features

If “Send As” is failing, try using “Send on Behalf Of” as a temporary workaround until Microsoft resolves the issue.

Step 3: Monitor Service Health

Keep an eye on Microsoft’s Service Health Dashboard in the M365 Admin Center for updates related to this incident or similar issues.

Step 4: Contact Support

Open a ticket with Microsoft Support to report your specific case and request further assistance.

Step 5: Communicate Internally

Inform your team about the issue and provide guidance on alternative workflows until normal functionality is restored.

The inability to use “Send As” permissions can disrupt critical operations for businesses relying on shared mailboxes for customer service or internal collaboration. It’s more than just an inconvenience—it impacts efficiency and customer satisfaction.

Questions That Remain

Has Microsoft officially acknowledged this issue beyond user reports? Are there broader implications for Exchange Online’s reliability following these back-to-back disruptions?

While Microsoft works on resolving these problems, administrators must stay proactive by monitoring permissions and implementing workarounds where possible. These disruptions highlight the importance of having contingency plans in place for cloud-based services.

If you’re experiencing similar issues or have found solutions, share your insights with others—it’s a collective effort to navigate these challenges effectively.