Hi,
The confirmation email is a Joomla option, not an HikaShop one.
Please configure the Joomla user registration options.
For the guest checkout in HikaShop, it is not possible to do a guest checkout with an email of an already register user (because he can login with his existing account and not do it as guest user).
You have to use email which are not already used by your existing users.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.