Encrypting your email to keep it from prying eyes? As far as the NSA is concerned you're no different from a criminal.
Even if your emails were intercepted "inadvertently"—fairly likely given how wide a net these data gathering operations cast—the NSA can "retain and make use" of them "if they contain usable intelligence, information on criminal activity, threat of harm to people or property, are encrypted, or are believed to contain any information relevant to cybersecurity."
That is: Encrypted emails are considered equivalent to emails with "threats of harm to people or property."
This is just one of the rules that govern the NSA's intelligence gathering operations, as revealed in two top-secret documents published by The Guardian:
The Guardian is publishing in full two documents submitted to the secret Foreign Intelligence Surveillance Court (known as the Fisa court), signed by Attorney General Eric Holder and stamped 29 July 2009. They detail the procedures the NSA is required to follow to target "non-US persons" under its foreign intelligence powers and what the agency does to minimize data collected on US citizens and residents in the course of that surveillance. [...]
The top secret documents published today detail the circumstances in which data collected on US persons under the foreign intelligence authority must be destroyed, extensive steps analysts must take to try to check targets are outside the US, and reveals how US call records are used to help remove US citizens and residents from data collection.
Among other things, the documents reveal how even data pertaining to or coming from U.S. persons can be gathered, retained and used, and The Guardian writes that they seem to indicate that the NSA can "access Americans' call or email information without warrants."