To answer your questions: Yes, I have an account configured as outbound address. Yes, the account is correct. As far as "Use only this server" I could do that for the situation when I am emailing a batch or set of invoices all at once. That is, I could set it up for the biz address I need, do my invoice mailing, and then flip it back to Send Messages from....Account of selected mailbox" which is what I need for day-to-day emailing.
I have not totally solved this issue but here's what I did today: instead of having the email get docked in the Drafts folder, I just had QB send it. In this way the email address I selected in QB was honored.