How to send SMTP emails ?

Hi @Julian Somesan

Thank you for posting your question in Microsoft Q&A. 

As you description, looks like you are encountering: 535: 5.7.3 Authentication unsuccessful. For this error, on the Exchange online side, in addition to enabling authenticated SMTP submission, you could follow these steps to troubleshoot it: 

1.Use Exchange Online PowerShell to verify that authenticated SMTP submission (also known as SMTP AUTH) is enabled on the licensed mailbox that the printer or application is using to connect to Microsoft 365 or Office 365: 

In Exchange Online PowerShell, replace <EmailAddress> with the email address and run the following command: 

Get-CASMailbox -Identity <EmailAddress> | Format-List SmtpClientAuthenticationDisabled  

If the value is True, replace <EmailAddress> with the email address and run the following command to enable it: 

Set-CASMailbox -Identity <EmailAddress> -SmtpClientAuthenticationDisabled $false

2.Disable multifactor authentication (MFA) on the licensed mailbox that's being used: 

• In the Microsoft 365 admin center, in the left navigation menu, choose Users > Active users. 
• On the Active users page, choose Multi-Factor Authentication. 
• On the multi-factor authentication page, select the user and disable the multifactor authentication status. 

3.Exclude the user from a Conditional Access policy that blocks Legacy Authentication: 

• Sign in to the Azure portal as a Security administrator or Conditional Access administrator. 
• Browse to Microsoft Entra ID > Security > Conditional Access. 
• In the policy that blocks Legacy Authentication, exclude the mailbox being used under Users and Groups > Exclude. 
• Select Save. 

You can refer more via: Error: Authentication unsuccessful. 

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