The header of your Compose window always has To, Subject, and Cc fields. In addition, Bcc is available via the Header menu. You can add custom headers to the Header menu by adding them to the Custom Fields scrolling list on the Compose Window Properties category.
Some useful headers that you might want to use are the following:
Precedence: junk -- If the mail system cannot send a message with this header, the message will not be "bounced" back to you. This is handy if you are sending out a message to a large alias, and you do not wish to receive notices whenever the message cannot be delivered to an individual.
Type the label for the field in the Header Field line of the Properties window.
Do not type a colon following the label. The colon is automatically provided by Mail Tool.
If you want the field to be filled in with a default value, type this value in the Default Value field.
Choose either Before or After from the Add menu to add the new header field to the scrolling list.
To apply the change to Mail Tool, click SELECT on the Compose Window Properties Apply button.
To include the new header field in a mail message, choose the item from the Compose window Header menu, as shown in the menu at the left.
Figure 4-37 shows an example of a custom header field added to the Compose window.
To delete a custom field, click SELECT on the item in the Custom Fields scrolling list, click SELECT on the Delete button, and click SELECT on Apply.
To change a custom field, click SELECT on the item in the Custom Fields scrolling list, type the new values in the Header Field and Default Value text fields, click SELECT on the Change button, then click SELECT on Apply.