Snowwrestlers point is interesting, why is a warrant needed for a phone tap? You are clearly enlisting a 3rd parties help, and I think the general public knows that their phone company can listen in on their calls... People are clearly sharing the content of the call as much as the number with the Telco. It seems odd that there are 2 standards here, particularly as it's the same transaction.
A warrant is needed because there are explicit wiretapping laws that protect the content of phone communication. This was not a legal principle that was bootstrapped from an interpretation of the fourth amendment.
There may be explicit laws for wiretapping phones, but the expectation of privacy in email does rest on the 4th Amendment. Here's an example of a ruling on emails that are held by a 3rd party:
So there does seem to be a weird double standard in the "3rd party doctrine" between content and routing information.
I think if you asked 100 people on a random street what part of email they expect to be private, almost everyone would answer "all of it." I agree with the EFF that the Smith ruling is outdated.
