To create a new or edit an existing disclaimer you will need to login to the fortimail control panel, a guide can be found here. 

Once logged in you will see the menu structure on the left, the disclaimer can be found under System, Mail Settings and then Advanced Settings;

Using the drop down menu you can select whether to

Disable - don't use a disclaimer,

Use system settings - use a disclaimer written by Internet Central,

Use domain settings - write a disclaimer specific to your domain.

If you choose to use domain settings you can specify what options to enable, for both incoming and outgoing messages.

The message header is a section of the mail which is hidden for normal use, however this all the servers the mail has passed through, the scores subjected to the mail and any delays.

The message body is the actual message, this is viewable by the recipient, usually at the end of the message.

To save the disclaimer to the system, click return.