There is not a preview before emailing estimates invoices etc to be able to change the recipients email address.
If you use send forms under file then you can change the recipients details for each document otherwise no it doesn't give you a preview if you use the email button on the invoice etc screen, just uses the last saved address. If you don't save the email addresses it will ask you for an address when you click to be emailed.
I usually send thru send forms which asks if you want to save the email address, I see it doesn't do that when sending from invoice screen. Do you use Outlook to send forms? I just had a play around on ours & it does give us a preview in outlook before sending but it didn't in 2017 version, we had issues with the email but that seems to be sorted in 2018. Another thing to check is, in preferences under online company preferences is display each email before sending checked? One of our other company files it wasn't checked & that was the one we weren't getting previews on. Hope this helps.