I now have this problem - as of last week.  I cannot send invoices from QB's via my usual Yahoo account without going to my yahoo security settings, copy the long code, paste it into QB's instead of my usual pw, then send the invoice.   I must do it EVERY time!!  what happens when I send my annual bulk invoices??? 400 of them in one day.   I have spent hours with Inuit AND with yahoo!  NO ONE claims to understand the problem except the users posting here.  WE cannot do our work!