An email on your iPhone will display a picture of the sender, with useful profile info from LinkedIn. To use the service, a LinkedIn user must route all of their emails through LinkedIn’s ‘Intro’ servers, which will right in your emails. LinkedIn also sniffs the contents of users’ iOS calendars, including sensitive information such as confidential meeting notes and call-in numbers, which they then transmitted in plain text, not encrypted. LinkedIn said that, during installation, the servers temporarily cache your password in order to add a new Mail account.
Source: https://thehackernews.com/2013/10/linkedin-intro-ios-app-can-read-your.html