When I delete my email on my BB it does not delete the email on my exchange server. Also, I cannot reply to emails sent to my Yahoo account, why is that?
I can't speak to the Yahoo issue, however when you delete emails, you should be prompted whether you want to delete the emails from just your BB, your BB and your email server, or cancel the operation. If not, from the email application select Options -> General Options -> Confirm Delete -> Yes. If this is set to No, I believe (someone will correct me if I'm wrong email will only be deleted from the BB. This works for BIS, at any rate. There's another thread in this forum that talks about email reconciliation and BES, so you might want to check that out as well.
For the Yahoo account, are you able to COMPOSE and SEND a new email from the Yahoo account address... at the top of the email it would say, SEND USING [email protected].