Sign Up Email can't be sent out

-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.5
-- PHP version -- : 5.6
-- Browser(s) name and version -- : Chrome 50
-- Error-message(debug-mod must be tuned on) -- : There was an issue with the email sending process, so the activation link has not been sent to the e-mail address you entered. Please contact the administrator.

Dear all,

I upgraded Joomla 3.3 to 3.5 and found that the sign up page after filling out user info shows--

Page not exists
0 Invalid address:

So I upgraded Hikashop from 2.5 to 2.6.3, the pages shows this instead---
There was an issue with the email sending process, so the activation link has not been sent to the e-mail address you entered. Please contact the administrator.

New User doens't get email notification for opening up the account.

What can I do now? Thank you so much.

Thank you, Nicolas. It seems work now.