From the good people at riseup.net.
OK, so you are using a corporate email provider. You aren’t bothered
that they mine your behavior to better sell your eyeballs to advertisers. One problem, however, is that governments around the world are using these private corporations as a vast multiplier of state power. The big email providers won’t disclose how many requests for data they get from the government. Yahoo, for example, has said in court that they should not be forced to release this information because it would be too shocking for their customers .
That sounds bad. But it gets much worse. The Wall Street Journal, among others, has reported on how the US government uses email traffic to
build a map of the relationship between everyone in the country:
“According to current and former intelligence officials, the [NSA] now
monitors huge volumes of records of domestic emails… [the telecoms] are giving the government unlimited access to a copy of the flow of
[internet] communications.” 
The good news is that there are easy things you can do to prevent corporate and state surveillance. The most important step is to use a secure email provider. For more information, see:
 Gorman, Siobhan. 2008. “NSA’s Domestic Spying Grows As Agency Sweeps
Up Data: Terror Fight Blurs Line Over Domain; Tracking Email.” Wall
Street Journal. http://online.wsj.com/article/SB120511973377523845.html
(depending on where you are, the url might not work, but there are many
mirrors of the article)