Zoho support Can not send invitation email to portal user

Zoho support Can not send invitation email to portal user

I try to use Zoho support but have a problem. I don't know why.
- When a customer sign up at the portal user they don't receive the invitation email.
- When I go to contact manu, choose 1 user and click invite --> they don't receive the invitation email.

So the user don't get the password to log in portal user.

Anybody can fix, Please help me.

Thanks so much.