Send & Receive Issues

How to Fix SendAsDenied Error When Sending SMTP Email from Exchange Alias

Maira Mehtab Oct 15, 2026 870 views

Question details

The user needs to send SMTP emails from an Exchange Online email alias without encountering a SendAsDenied error.

Fix SendAsDenied Error When Sending Email Through SMTP from an Exchange Alias

Product: Exchange Online
Device & OS: not provided
Scenario: Attempting to send an email using an alias address via SMTP while authenticating with the primary Microsoft 365 account credentials.
Observed behavior: The system produces a SendAsDenied error because the alias lacks independent authentication and the primary account is restricted from sending via the alias over SMTP without explicit permissions.

Before you start

Ensure you have global administrator or Exchange administrator access to the Microsoft 365 Admin Center to modify mailbox delegation and mail flow settings.

Solution 1Recommended

Enable Alias Sending and Grant Send As Permissions

Since an alias is not a separate identity, you must enable alias sending globally and ensure the primary mailbox has explicit Send As permissions.

Exchange Online aliases function as proxy addresses rather than independent accounts. To send emails from them via SMTP, the authenticating account must have explicit permissions, and tenant-wide alias sending must be enabled.

Enable sending from aliases

Sign in to the Exchange Admin Center. Navigate to Settings, select Mail flow, and check the box for 'Turn on sending from aliases'. Save your changes.

Locate target mailbox

Go to Recipients > Mailboxes in the Exchange Admin Center and select the mailbox or recipient that owns the alias address.

Grant Send As permission

Click on the Delegation tab. Under the 'Send as' section, add the primary account that will be authenticating the SMTP connection.

Wait for propagation

Allow up to 60 minutes for the permission changes to propagate across Microsoft 365 servers before testing the SMTP connection again.

Enable Alias Sending and Grant Send As Permissions

Permission Propagation: If you still receive the SendAsDenied error immediately after making changes, wait a bit longer. Exchange Online permissions often take time to fully sync.

Solution 2

Create a Separate Mailbox for Independent Authentication

If your application strictly requires the address to have its own independent SMTP password, you must convert the alias into a standalone mailbox or mail-enabled user.

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Frequently Asked Questions

Why does my Exchange alias throw a SendAsDenied error in SMTP?

An Exchange alias is merely a routing address, not a separate user identity. It lacks its own password, so SMTP protocols will reject the send attempt unless the authenticating primary account is explicitly granted 'Send As' permissions for that address.

Can I assign a unique password to a Microsoft 365 email alias?

No, an email alias cannot have its own independent password. If you need dedicated login or SMTP authentication for a specific address, you must set it up as a separate user account or a shared mailbox.

How long does it take for 'Send As' permissions to take effect in Exchange Online?

Permission changes in Microsoft 365 Exchange Online typically take anywhere from 15 to 60 minutes to propagate through the system, though in rare cases it can take up to 24 hours to fully sync.

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