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 and select whether to send notifications when the action is
taken.
Under External Disclaimer section, select
Enable External Disclaimer.
If required, modify the text in the External Disclaimer
Content text field.