Unwanted Email Header: "This message contains unscannable attachments that could not be verified virus free. Open only if you were expecting this message". Appeared recently.

Question

Hi, 
 Wondering if anyone can help. I have been looking at posts and I don't seem to have found the answer. In the last week a client of mine has been receiving this header on their emails: 
 'This message contains unscannable attachments that could not be verified virus free. Open only if you were expecting this message.' 
 They only seem to be getting this message when people email eternally, and not internally. A few have mentioned a setting within Sophos, but for the life of me I cannot find this setting. If anyone can provide the steps I would be very grateful. Is this header generated from Sophos even? or through Microsoft? I'm guessing as a few threads have been created about this, it relates to Sophos. 
 Hope you can help. Thanks in advance. 
 Below is an example of the message:

Jaydeep · Answer

Hi Ryan Knighton 
 This banner is added as per configuration in Sophos Email Appliance. Please check the settings under Configuration > Threat Protection > Unscannable Attachments to all (Policy Rule). If this is wrongly added to genuine emails, the likely cause would be the AV data expiring in the appliance and therefore the device cannot scan any mails. Please check if the AV data is updated in the device, if not, correcting it would resolve this.