Just to help any others out who may be experiencing the same symptoms – I had a problem where I was unable to hit ‘Send’ when replying or sending using Yahoo Mail (through IE) of late. I kept getting a script error indicating that AC_SelectedEmails was not found. As a result I could not send or reply.
The answer: you need to disable the macromedia flash add-in. Go to Tools, Manage Add-Ons… and in the dialog select the Shockwave Flash Object and choose the disabled option). You will get a warning icon telling you that an add-in that IE tried to use was disabled when you log-in to mail, but don’t reenable it. Your email should work fine after that.
It may be that the Shockwave Active X control just needs refreshing, I’ll play around and let you know.