The NSA was storing bulk plaintext emails from (at least) Microsoft, as of the time of the Snowden leaks. Microsoft actively assisted them.

https://www.theguardian.com/world/2013/jul/11/microsoft-nsa-... ("Microsoft handed the NSA access to encrypted messages")

> "Microsoft helped the NSA to circumvent its encryption to address concerns that the agency would be unable to intercept web chats on the new Outlook.com portal;"

> "The agency already had pre-encryption stage access to email on Outlook.com, including Hotmail;"