StartTLS encrypts emails as they transit between sender and receiver and is designed to prevent snooping. It also ensures that emails you receive from Twitter havent been read by other parties on the way to your inbox if your email provider supports it. We commend those email providers like Gmail & AOL Mail that have turned on TLS and we ask all other providers that haven’t yet to prioritize it. Together, we can protect the privacy of every user. Together, it can protect every users privacy.”]
Source: https://blog.twitter.com/engineering/en_us/a/2014/greater-privacy-for-your-twitter-emails-with-tls