At first I thought "shares information with third parties and that they can [copy your email]" (emphasis mine) meant that the third parties can see & copy your email. I can't find anything like that language in Nylas' privacy policy (https://nylas.com/privacy-policy/) and so I guess your "they" meant "Nylas". Given their precise business model is making synchronized copies of your email inbox, that doesn't sound very unreasonable to me.