I cant add mailbox to user in mail admin

I cant add mailbox to user in mail admin

1. Im login as ivhanfg@andalin.com
2. I request mailbox to admin (pict 1)
3. Admin got request and link for create mailbox ivhanfg@andalin.com
4. I cant add mailbox for ivhanfg@andalin.com. Display warning : "Given emailid already exist"

Can u delete ivhanfg@andalin.com at ur system ? and i will create again in mailadmin. I think it will solve my problem.