So LinkedIn built an API that "extends" the iOS mail app. Using a proxy server that speaks the IMAP protocol that most email providers employ, Intro tacks the banner featuring the sender's LinkedIn information onto the top of the message while the message is being forwarded from the device to the email provider. In this process, it also recognizes the specific device used to open it, and adapts its banner to meet its requirements. Then, using a CSS code response that is native on the mobile version of Safari, LinkedIn was able to make the Intro bar interactive, meaning users can click on it to find out more information about the sender."
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