Re:Sender ID, SPF, DomainKeys (Score 1)

Protecting the envelope sender protects from bounces being sent to a spoofed sender, though there are other ways to do this on the sender's end.

Potentially SPF, Sender ID and DomainKeys could be used in concert since there are no conflicts between them with the exception of Sender ID's re-use of SPF v1 records that may not have been written with Sender ID's PRA in mind.