You can configure mail header size or configure ScanMail to add a
disclaimer at the top of message body of all the incoming messages from external
domains.
Procedure
Click Administration → Mail Settings.
The Mail Settings screen appears.
Under Mail Restrictions section, do the following:
Maximum size of each mail header: specify the
header size between 1 and 10MB.
Action on mail exceeding header size limit: select
an action from the drop down menu.
Under External Disclaimer section, select
Enable External Disclaimer.
If required, modify the text in the External Disclaimer
Content text field.