I currently use FusionInvoice, and plan to convert to InvoicePlane when v2 is stable.
It would be nice to be able to setup multiple smtp destinations, and allow a choice of smtp to be made for each client email address.
The reason is that my invoices tend to go to spam folders, but it helps if I send from the same provider that a client uses. For example, sending to a client with a gmail email address, I would like to send from my gmail address instead of my business address so it goes to their inbox instead of to their spam folder.
My regular business emails go to their inbox, but something about the invoices makes them to go spam sometimes.