How to Configure SMTP AUTH with STARTTLS on Third-Party Devices
Question details
Users need to properly configure SMTP AUTH with STARTTLS on a third-party device to successfully send Microsoft 365 notifications.
- Product
- Microsoft 365 Exchange
- Device & OS
- not provided
- Scenario
- Setting up a third-party phone system or device to send email or voicemail notifications via Microsoft 365 SMTP.
- Observed behavior
- The device fails to send Microsoft 365 notifications when STARTTLS is selected due to certificate, port, or authentication mismatches.
Ensure you have global administrator access to your Microsoft 365 tenant and the administrative login credentials for your third-party device or phone system.
Verify SMTP Endpoint, Port, and TLS Certificate
Ensure the third-party device is using the correct Microsoft 365 SMTP settings and trusts the required TLS certificate authority.
Microsoft 365 requires specific port and encryption settings for SMTP client submission. Additionally, the device must inherently trust the Microsoft 365 certificate authority to establish a secure STARTTLS connection.
Access your device's network or email settings and set the SMTP server exactly to 'smtp.office365.com'.
Change the SMTP port to 587 and ensure the encryption method is explicitly set to 'STARTTLS'.
Enter the full Microsoft 365 email address and password of the licensed mailbox you intend to send from.
Check the device's security settings to confirm it trusts global public Certificate Authorities (CAs). Ensure the device firmware supports TLS 1.2 or higher.
Escalate to Microsoft 365 and Device Manufacturer Support
If configuration settings are correct but the connection fails, a backend trace or firmware update may be required.
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Frequently Asked Questions
What is the correct SMTP server and port for Microsoft 365?
The correct SMTP server endpoint for Microsoft 365 is smtp.office365.com, and you must use port 587 with STARTTLS encryption enabled.
Why is my third-party device rejecting the STARTTLS certificate?
This usually occurs if the third-party device's firmware is outdated and lacks the latest root certificates, or if it does not support TLS 1.2 or higher, which is required by Microsoft 365.
How do I enable SMTP AUTH for a specific mailbox in Microsoft 365?
Go to the Microsoft 365 admin center, navigate to Users > Active users, select the target user, click on the Mail tab, select 'Manage email apps', and ensure 'Authenticated SMTP' is checked.
