It is not about just sending the email.
Postfix can not manage high queue load.It doesnt have any reputation managemant.No bounce or fbl processing.
It simply a free software for starters.No serious email marketer would use it if they know what they are missing.
you can all do it with IEM + Plugins with Postfix. You can increase the volume with several server and Loadbalancing.
Sending 8Millions email in less than 24h with that's setup and don't need to buy pmta or buy a cracked version some want sell to me when looking for other solution.
IEM plugins are not cheap too.
I'm in Email markerting since 1998. Run long time Lyris one but they turn very expensive and was certainely before the fastest SMTP server.
However they change business model about 7 years ago if i'm remember well.