So i have setup a gmail account, which you can do for free, then change the sending address to the gmail account for all invoices and then it sends them from gmail rather than the quickbooks server. Avoiding the blacklisting on QBO emails. Not ideal, but it does work and will save you from having to do all the other work arounds.  All the best and good luck.

1. If you have a gmail or G-suite email account, you can use the Send with Gmail feature to send invoices from your own address
2. If you don't have a gmail address  You can set up a free account and use that account to send with Gmail feature to send invoices from your own address