According to this article => http://www.howtogeek.com/166507/why-most-web-services-dont-use-end-to-end-encryption/ Gmail doesn't store the encryption key. But Gmail reads whole of users' email for advertisement or other purpose like government! (Based on Mr.Snowden disclosure)
The idea of “end-to-end encryption” — you could also refer to it as “local encryption and decryption” — is different. With end-to-end encryption, the data is decrypted only at the end points. In other words, an email sent with end-to-end encryption would be encrypted at the source, unreadable to service providers like Gmail in transit, and then decrypted at its endpoint. Crucially, the email would only be decrypted for the end user on their computer and would remain in encrypted, unreadable form to an email service like Gmail, which wouldn’t have the keys available to decrypt it. This is much more difficult.
Is gmail or the other mail services doing this ? (Storing encrypted data in the database without storing it's private key)