[ctz]

You really think google adds private emails to its public index? Get real.

[angusgr]

If the URL is a publicly accessible webserver with no robots.txt telling them to stay off it, I wouldn't be surprised if it gets fed to the crawler.

[eli]

Google can even index files listed in robots.txt (it just doesn't crawl them)

[andymurd]

The email wasn't indexed but (maybe) a link in the email was.

Google honours robots.txt, X-Robots headers etc but everything else is fair game.

[rahoulb]

They scan my private emails to target advertising at me - why would they not follow links (as the link obviously denotes something I'm interested in)?

And, as the others state, if it's not robots.txt denied then why not add it to the public index?