Mail Add-On for CRM

So, I'm a little confused. I use Zoho mail for my company (1 man show) and Zoho CRM (free edition). I would like to add the mail add-on but I am not sure about the pricing. Is it $5 / month per user for the free account or do I need to upgrade my CRM account to professional account at $25 / month (billed monthly)? Which one is it? $5 / month for the the add-on or $25 / month for the upgrade? I just need the email add-on for my CRM and everything else in the free edition is fine for me so I obviously don't want to upgrade to a Pro CRM account just to get the email add-on feature...