They advertise it as such.
From their privacy policy:
"Incoming messages are scanned for the purpose of spam detection unless you disable spam protection for your account... To make message searching fast, we build an index of your messages (this is a table, just like you would find at the back of a reference book, in which you can look up a word to quickly find the emails in which it appears).
No information from any of these activities is used for any other purpose, or to compile any kind of profile on our users."