Mailing List emails to digest subscribers are grouped together so that multiple emails are combined into one digest email. In order for headers and such to function, they must be enabled in the Digest Settings page for the list. The digest email is formatted as shown below.

DIGEST HEADER
  Email 1
DIGEST SEPARATOR
  Email 2
DIGEST SEPARATOR
  . . .
DIGEST SEPARATOR
  Email n
DIGEST FOOTER

To edit the digest header, footer, or separator, choose the corresponding option from the manage pop-up menu on the Mailing Lists page.