office 365 Failed to send a test message using SMTP Service

Situation: The client has QNAP storage. They can’t send notification with this error: Failed to send a test message using SMTP Service.

Troubleshooting: 1. SMTP settings is correct and test is fine.

Device or Application setting	Value
Server/smart host	smtp.office365.com
Port	Port 587 (recommended) or port 25
TLS/StartTLS	Enabled
Username/email address and password	Enter the sign in credentials of the hosted mailbox being used

2. However, test sent to Recipient failed.

3. We find Microsoft article, “How to set up a multifunction device or application to send email using Microsoft 365 or Office 365” mentioned: “Option 1 (recommended): Authenticate your device or application directly with a Microsoft 365 or Office 365 mailbox, and send mail using SMTP AUTH client submission. This option is not compatible with Microsoft Security Defaults or multi-factor authentication (MFA). If your environment uses Microsoft Security Defaults or MFA, we recommend using Option 2 or 3 below”.

$. the option 2 and option 3 are “Option 2: Send mail directly from your printer or application to Microsoft 365 or Office 365 (direct send)” and “Option 3: Configure a connector to send mail using Microsoft 365 or Office 365 SMTP relay”.

4. We use option 2 fixes the problem. Here is the settings

Device or application setting	Value
Server/smart host	Your MX endpoint, for example, contoso-com.mail.protection.outlook.com
Port	Port 25
TLS/StartTLS	Enabled
Email address	Any email address for one of your Microsoft 365 or Office 365 accepted domains. This email address does not need to have a mailbox.

We recommend adding an SPF record to avoid having messages flagged as spam. If you are sending from a static IP address, add it to your SPF record in your domain registrar’s DNS settings as follows: