Do you ever get Non-Delivery Report (NDR) notification emails that have "Access Denied" when you try to send an email to the Sophos Email protected domain?
Here is how the NDR looks like:
This is usually caused by the following:
1. The mailbox for the target recipient does not exist in the 'Manage Protection > Mailboxes' page. Make sure it exists within Sophos Email.
2. The MX record for the recipient domain is not specified or pointed to the wrong Sophos region. The correct MX record is specified at 'Configure > Settings > Configure External Dependencies > Inbound settings tab' as per below:
NOTE: If both of the above are confirmed correct and existing but the issue persists, then please get a support case created so it can be further investigated by our engineers.