Testing SMTP Auth

https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365#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

When setting up SMTP Auth, ensure that the target mailbox is configured for smtp relay. You can enable SMTP authentication for the target mailbox by 

User --> Mail --> Manage email Apps --> check Authenticated SMTP

Then open an elevated powershell and run the script below to test

$msolcred = get-credential

Send-MailMessage –From SenderEmail –To SeceiverEmail –Subject “Email via PS” –Body “I am Testing the SMTP Relay Service from PowerShell” -SmtpServer smtp.office365.com -Credential $msolcred -UseSsl -Port 25