Forwards Defaults
By default, you CANNOT auto-forward MS365 emails externally as of September 2020.
Three settings are available:
Automatic: Automatic external forwarding is blocked. Internal automatic forwarding of messages will continue to work. This is the default setting.
On: Automatic external forwarding is allowed and not restricted.
Off: Automatic external forwarding is disabled and will result in a non-delivery report (also known as an NDR or bounce message) to the sender.
More details in https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/external-email-forwarding?view=o365-worldwide
Default setting will block an attempted external forward and yield this error:
5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS(7555)