Microsoft 365 - SMTP authentication unsuccessful error when sending emails – Mailbird

🚨 Problem

With Microsoft 365 accounts, the "Authentication Unsuccessful" error can occur when a user tries to send emails while IMAP is selected as the server type. This issue arises because SMTP authentication is blocked by default in Microsoft 365 settings for enhanced security. This is Microsoft's decision, and it is entirely beyond our control. Organizations using Microsoft 365 must therefore enable this setting.

To avoid such issues, we strongly recommend using the Exchange protocol instead, which is available in our Premium plan.

🛠️ Solution

While using IMAP is still possible, fixing the "Authentication Unsuccessful" error requires that Authenticated SMTP be enabled in your organization’s Microsoft 365 settings (as mentioned above).

💡 Tip: Only an Administrator can enable this setting, either through the Microsoft 365 Admin Center (for individual users) or via PowerShell (for the entire organization).

Important Note for GoDaddy Users

If your Microsoft 365 account is hosted by GoDaddy, you won’t be able to enable SMTP for individual users. Instead, SMTP must be enabled for the entire organization via PowerShell, as GoDaddy accounts do not have access to the Microsoft 365 Admin Center. Once again, this is entirely beyond our control.

🧭 Steps to Enable Authenticated SMTP for a Specific User

Go to the Microsoft 365 Admin Center.
Navigate to Users > Active users and select the desired user.
Go to Mail > Manage email apps.
Ensure Authenticated SMTP is enabled.
Remember to save your changes.

💻 How to Enable Authenticated SMTP for the Entire Organization

Log into your organization’s Microsoft 365 environment via PowerShell.
Run this command: Set-TransportConfig -SmtpClientAuthenticationDisabled $false

📚 Additional Resources

For more detailed instructions, please refer to Microsoft’s official documentation:
Authenticated client SMTP submission in Exchange Online.

💬 Need More Help?

Although we can only provide very limited assistance in this area (given that much of these problems are caused by settings which are beyond our control), you're welcome to contact our Customer Happiness Team if you would like further guidance. We’ll do our best to help you.