Hi, I am using yahoo mail for many years now.
I was thinking that if you can integrate your messenger which by far is the oldest and coolest at that time with your email. i.e Microsoft has skype in the mails, Google has hangout in the chat. Why not you market the messenger and put that inside the mailbox.
At that time there were millions of users who use only chat and rooms to communicate, which will give you the lead as mostly now do not use messengers to get them back.