I have a Fenix 5S and previously a Forerunner 630 which shared the same problem.
My watch is connected to an iPhone and I receive Smart Notifications. Whenever I unlock my phone, I suddenly get about 10 e-mail notifications on my watch, even though the e-mails are old e-mails which I have already read on my phone. I use the normal Mail app on the iPhone which fetches e-mails from my Gmail account every 15 mins.
It's quite annoying. Does anyone else have this issue?