Sending administrative Mails with Office365 through SMTP fails

Question

When trying to send administrative mails using Office365 SMTP endpoint fails. What can I do to fix this?

Answer 1

You should not use the SMTP endpoint, but the MX endpoint. 
This is the recommendation from Microsoft: 

"This indicates that you are connecting to the SMTP client submission endpoint (smtp.office365.com), which can't be used for direct send. For direct send, use the MX endpoint for your Office 365 tenant"

More info: 

English: 

https://support.office.com/en-us/article/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-office-365-69f58e99-c550-4274-ad18-c805d654b4c4?ui=en-US&rs=en-US&ad=US

German: 
https://support.office.com/de-de/article/einrichten-eines-multifunktionsger%C3%A4ts-oder-einer-anwendung-zum-senden-von-e-mails-mithilfe-von-office-365-69f58e99-c550-4274-ad18-c805d654b4c4?ui=de-DE&rs=de-DE&ad=DE