I'm having the same issue as imagemarket, and went through the steps provided earlier in this thread, and then through those provided by ShallyMarR.

Nothing has worked.Invoices still get sent via a former employee's email account no matter what settings I put in Quickbooks, IE, and Outlook.

Calling or messaging with Customer Support has been frustrating and wasteful, and this isn't the kind of issue that should require someone to purchase a support plan. This is a bug issue in my view, pure and simple.

I will say this: In my situation, my Outlook was having a total meltdown Monday morning. My mail server's tech support advised me to delete my Outlook account and then re-install it. This issue with QuickBooks seems like it could be a side effect of that (among others).