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smtp server requires a secure connection or the client was not authenticated office 365

Hi microsoft support,

I was trying to connect a third party management software (that can send emails to other people) to office 365 smtp.

But it gives me the following error:

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smtp server requires a secure connection or the client was not authenticated 

The server response was: 5.7.57 Client not authenticated to send mail.

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What could be the problem? Do I need to allow third party apps to have permissions to access Office 365? How do i do it?

Regards.
João Moreira

Hi João,

May I double confirm which method are you using, is it SMTP client submission or Office 365 SMTP relay?

However, based on the error it indicates that your application connect to the SMTP client submission endpoint (smtp.office365.com). Please make sure that your application must also logon to a mailbox to send a message and support TLS version 1.2 or above.

This error occurs when you have not entered mailbox logon credentials in the application settings. If there is no option to enter credentials, this application may does not support SMTP client submission. For your reference: Fix issues with printers, scanners, and LOB applications that send email using Microsoft 365 or Office 365

Moreover, please use Office 365 SMTP relay option to check this issue. For detailed information see under Option 3: Configure a connector to send mail using Microsoft 365 or Office 365 SMTP relay

Your understanding and cooperation are highly appreciated. Have a nice day and stay safe!

Best regards,

Shyamal

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How can I check which method is used?

The app maker told that could probably be SMTP client submission.

It's an external app that uses .net email sending script. 

All configurations are correct, if we use Gmail we have to check option to let "less safe applications" in order to work. Is there any kind of permission in this kind of email? 
It has STARTTLS enabled, tried port 587 and 25 and gave the same error.

Best regards,
João Moreira

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Hi João,

Thanks for the reply. You may refer to the article to check the method options: How to set up a multifunction device or application to send email using Microsoft 365 or Office 365

You may verify that SMTP AUTH is enabled for the mailbox. See Enable or disable authenticated client SMTP submission (SMTP AUTH) in Exchange Online

However, there is no such "less safe applications" option setting in the Office 365. If you enabled multi-factor authentication (MFA) please disable it, since SMTP AUTH client submission method option doesn't compatible with (MFA).

If you already tried suggestion above and still having issue then please use Office 365 SMTP relay (Option 3) to check this issue.

Your patience is highly appreciated.

Best regards,

Shyamal

---------------------
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* We are happy and always here to help you, and share the Microsoft 365 for business online information with you.

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Last updated September 9, 2024 Views 5,532 Applies to: