Hello,
I've been able to set my default send account in the past, but it's not working now. I can set it (to the email I want). However, the default will be that email only if I'm composing it from scratch, or if I've received an email to it, and am replying. If I've received an email from the other account, when I hit reply, it defaults to the email that received the email. So, basically, it will not follow the default email rule --when replying to an email received by any but the default account.