Exchange Connection Problems

How to Fix SMTP 5.7.3 STARTTLS Required Error in Microsoft 365

Maira Mehtab Sep 22, 2026 870 views

Question details

A third-party application is unable to send emails using an Office 365 SMTP account and generates a STARTTLS requirement error.

Product: Microsoft 365
Device & OS: not provided
Scenario: Attempting to send outbound email through a third-party application integrated with an Office 365 SMTP server account.
Observed behavior: The email fails to send, and the application reports the error message: '5.7.3 STARTTLS is required to send mail.'

Before you start

Ensure you have access to your third-party application's email server settings, Microsoft 365 admin center credentials, and local network firewall configurations.

Solution 1Recommended

Configure SMTP Settings and Check Firewall Policies

Resolve the STARTTLS connection failure by verifying the correct Microsoft 365 SMTP settings and ensuring your firewall permits outbound traffic on the required port.

The '5.7.3 STARTTLS is required' error typically occurs when the client application fails to initiate a secure TLS connection with the Exchange server. This can be caused by incorrect port settings, missing authentication permissions in the Microsoft 365 tenant, or a network firewall blocking the secure handshake.

Update SMTP Server Details

Open your third-party application's email configuration panel and set the SMTP server to 'smtp.office365.com'.

Configure Port and Encryption

Change the outgoing port to '587' and ensure that the 'STARTTLS' (or TLS) encryption option is explicitly enabled.

Enable SMTP AUTH

Log into the Microsoft 365 Admin Center, navigate to the specific user's mail settings, and verify that 'Authenticated SMTP' is checked and enabled for that mailbox.

Adjust Firewall Policies

Check your local network or edge firewall settings to ensure that outbound traffic on port 587 is not being blocked. Modify the policy to permit this connection if necessary.

Firewall Check Complete: In many cases, once the firewall policy blocking the port 587 connection is corrected, the application will successfully complete the STARTTLS handshake and send emails.

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Frequently Asked Questions

What does the 5.7.3 STARTTLS error mean?

This error means the application is attempting to connect to the Office 365 SMTP server without utilizing STARTTLS encryption, or the secure connection is being intercepted or blocked by a network firewall before it can be established.

How do I enable Authenticated SMTP in Microsoft 365?

Go to the Microsoft 365 admin center, navigate to Active Users, select the specific user account, click on the Mail tab, and choose 'Manage email apps'. Ensure the 'Authenticated SMTP' option is checked.

Why must I use port 587 instead of port 25?

Port 587 is the standardized port for secure email client submission. Microsoft 365 requires port 587 coupled with STARTTLS encryption to guarantee that your login credentials and email contents are transmitted securely over the internet.

Can my local firewall really block STARTTLS?

Yes. If your local network router, edge firewall, or ISP blocks outbound connections on port 587, the application cannot establish a communication channel with the Microsoft 365 servers, resulting in a failure to initialize STARTTLS.

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