Two Office 365 accounts rejecting internal messages

[AndyT63]

Our school Office 365 domain has about 35 users.

We have just discovered that 2 users are bouncing internal emails.  If anyone tries to send an email to either of the users they get:

Delivery has failed to these recipients or groups:

xx (xx@staugustines.dorset.sch.uk)
Your message wasn't delivered because the recipient's email provider rejected it.

Diagnostic information for administrators:

Generating server: AS8PR01MB7606.eurprd01.prod.exchangelabs.com
xx@staugustines.dorset.sch.uk
Remote Server returned '550 5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS(7555)'

I have checked both of the users accounts and neither have any mail forwarding set.  And it is only affecting these 2 users.

Does anyone know what might be going on?

Andy

[#2]

Hi, this is most likely related to a new default setting in the outbound spam filter policy. It was recently announced via Message center. Go here for configuration https://protection.office.com/antispam

For more info
https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/external-email-forwardin...