[ietf-dkim] RFC 4871: Signature Expiration

[Florian Sager]

I wonder why the signature expiration date (x-param) was included in the 
signature itself.
If there was an optional expiration date contained in the _domainkey DNS 
entry besides the public key instead, a mail admin could react in the 
short-term to e.g. abuse of the according private key without 
interfering the validation of signatures before this expiration date.

Florian