For many header field names...
In draft-delany-domainkeys-base-01.txt [Page 15]
h = A colon separated list of header field names ...
(...)
Whitespace is ignored in this value.
Example)
h=Sender:Received:DomainKey-Signature:
Received:From:To:Subject:Date:Message-ID;
Logged In: YES
user_id=1048957
Added for 0.2.5.
Logged In: YES
user_id=1048957
v0.2.5 released.