The IRS may take your emails without obtaining a warrant first

[Feuer Frei!]

Originally Posted by Herr-Berbunch

How do they know they're targetting a US citizen's email address?

They check your ip address and match it against a IP-to-location database.

Oh and if you wanna know how to make your emails almost undetectable, read on:

The trick is to create a pseudonym private account and compose messages and never send them. Yep, save them in draft. The recipient logs in under the same account, read it and reply, without sending anything. The traffic would not be sent across the networks through Google's data centers, making it nigh on impossible for the National Security Agency or any other electronic signals eavesdropping agency (such as Britain's elusive GCHQ) to 'read' the traffic while it is in transit.
If you have a dynamic isp, even better.
Hell, just ask Gen. David Petraeus