I am pretty sure that Apple don't distribute malware in its Sierra installerI have never heard of mail opening by itself, you either have some kind of script running or an App running that wants to send an email and open the Mail App.
There is another reason it could be opened, malware but I never ever had Malware on my Mac and I use Macs for 16 years, started on MacOS 9, had OS X installed since 10.0 beta, so the chance is really really low.
I not aware of any malware opening an App either.