I use Hotmail and I set document seller to send an email with a dowload link.
this email (the confirmation order) goes to spam box directly.
This is a big issue.
What's from email ? and From Name ? What's the email which you entered on order form ? Maybe your server has a spam filter or something like that. Could you please try use a different email ?
Documents Seller use Joomla core function for sending emails and there is no settings to make it sending to Spambox . It is just something related to your email or your server .