unknown X-Foo header label in failure template -- ignoring this template

Today I tried to customize a bounce template with an additional

X-Foo: blah

header, but received the warning message from the bounce daemon:

warning: unknown "X-Foo" header label in failure template -- ignoring this template

Which headers are deemed worthy? Why can't I add an X-anything: header?

-- Ralf Hildebrandt Geschäftsbereich IT | Abteilung Netzwerk Charité - Universitätsmedizin Berlin Campus Benjamin Franklin Hindenburgdamm 30 | D-12203 Berlin Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962 ralf.hildebrandt_at_charite.de | http://www.charite.de

• This message: [ Message body ]
• Next message: Devdas Bhagat: "Re: Store Mail Headers?"
• Previous message: Robert Schetterer: "Re: Postfix client against Windows Exchange server"
• Next in thread: Wietse Venema: "Re: unknown X-Foo header label in failure template -- ignoring this template"
• Reply: Wietse Venema: "Re: unknown X-Foo header label in failure template -- ignoring this template"

• Contemporary messages sorted: [ By Date ] [ By Thread ] [ By Subject ] [ By Author ]