Yes, pretty much. There's a link at the top of the Faxing topic that will take you to the Email help for basically the same window.
If you look at the BillTo/ShipTo tab in the invoice you are wanting to email, the BillTo pulls the email address from the BillTo side of that tab. In my test, there was no email address there, so as a 'blank', the email was automatically placed on Hold in the Email Queue.
The Customer selection pulls from the Invoicing tab.
It's also important to note that the addresses used are those that were in effect at the time the document was created. Except for the Customer option. If you've subsequently changed the email address on the Invoicing tab and select Customer, whatever email address is currently shown on the Invoicing tab of the Custom record will be used.
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Regards,
Softrak Tech Support