Let me talk you through what I do.
In presets I have For Email and For Email ( Hard Disk)
If I select For Email then I don't get the "Post Processing " box
If I chose For Email ( Hard Disk) I do get the box but the "Export To" states Hard Drive.
I'm using Win 10.
I don't get the "Post Processing " box
Same with me!
You may need to do it through the Email Account Manager-
Right-Click a photo,
Select [Email Photo]
Open the Menu in the "From:" field
Choose- [Goto Email Account Manager]
You need to 'Add' you Email Client details in this dialog.
If you then click on the [Validate] button, and all is correct, Lightroom will show "Validated" and emails can then be sent.
Complicated- I know!!!