I used Exchange because it was what I most familiar with. SharePoint operates in similar matter with all sharding (though backend is still MSSQL with it's sharding last I checked)
Sure, PostFix/DoveCot will scale if you are doing just email. Once you add GroupWare requirements, PostFix/Dovecot are no longer in same boat.
SharePoint does not use [SQL] sharding. Each Site Collection is contained within a single Content [SQL] database. However the blobs themselves can be stored elsewhere via a provider, out of the box a file system provider is available (in SPO they use Azure Blob Storage).
Sure, PostFix/DoveCot will scale if you are doing just email. Once you add GroupWare requirements, PostFix/Dovecot are no longer in same boat.