Update: 

I believe I have found what was causing the issue for me. I set up a test invoice and sent it using the gmail I had logged in to and it was undelivered again. 

I then removed that email and set up another gmail address as the sender and this time I noticed when I connected it there was a check box I had to click next to the “give permission to send emails on your behalf” or similar wording. 

when I connected the first gmail it didn’t have this checkbox it just said I would be giving access but no checkbox anywhere. 

the invoice sent from the new email with the checkbox went through just fine. Not sure why but I don’t want to mess with anything anymore since it seems to be working now.