Propublica.org has a brief summary of the ways the government gets our data in No Warrant, No Problem: How the Government Can Get Your Digital Data. Here's what they say.
A showing of probably cause can get a warrant to bug our calls and get emails that are less than 180 days old. But there are easier ways to get emails.
Court order --
An assertion that info is relevant to a criminal inquiry can get a court order that will authorize the collecting of the phone numbers called or received in real time. The court order can also get them real-time access to computer IP addresses. Further, they can get opened emails and unopened emails older than 180 days without notifying the customer.
Administrative subpoena --
No court order needed for historical phone records or historical records of IP addresses. Opened and unopened emails that are at least 180 days old can be obtained with only a subpoena as long as they notify the customer whose email they've requested.
It's not clear why 180 days should make any difference or why opened emails are treated differently than opened emails. And in fact, bills are bouncing around both the House and the Senate that would require the government to get a warrant for any emails no matter how old they are. We'll see what happens.
Anyway, read the whole thing to get a more complete picture.