@Vasil Michev 


Thanks for the response.


The response you have given is the same as I receive from Microsoft when I discuss how services are limited for Guest users in a tenant, even though their business architecture is to use BYOD for both devices and subscriptions. It amazes me how much is overlooked from a Guest account, from little things like this to big things such as authenticating to IaaS and PaaS services.


It seems in this case the no-reply@sharepointonline.com is being treated as a spam email buy the Guests systems because it isnt passing authentication. I am exploring DMARC, DKIM and transport rule options to see which may be able to solve the issue in the safest way.