Email forwarding bounced (5.7.520)
Microsoft blocks external forwarding by default; connect the mailbox with the Microsoft sign-in instead.
What you see
A bounce saying "Your organization does not allow external forwarding" (code 5.7.520) after you set up forwarding from a Microsoft 365 mailbox to the OmniBrand forwarding address.
Why it happens
Microsoft 365's outbound spam policy blocks automatic forwarding to other domains by default, and some organisations cannot change it.
How to fix it
You do not need forwarding at all:
- Remove the forwarding rule in Outlook.
- On Accounts (/app/channels), connect email, enter the address and choose Sign in with Microsoft.
- For a shared mailbox, enter the shared address in step 2 and sign in with your own account that can open it; the account is created as the shared mailbox.
OmniBrand then reads the mailbox directly every minute and replies from it, with no forwarding rule.