We are also having issues sending invoices to multiple addresses.  I tried your way of using a comma and a space in between email addresses, however, it Still comes back as 'undeliverable' for both addresses.  Prior to us upgrading to QB 2020, we always used a semicolon ; and a space between addresses and it worked fine.  Now the email address to the right of the semicolon comes back 'undeliverable' ... I called and had an agent walk me thru some other steps, none of them worked.  This is crazy that we can't email multiple addresses anymore :( Ugh