Let me make sure I am understanding correctly, then I can give you some advice. Your users will be using Zimbra to send and receive email rather than the client in RedHorse. You have an account in Zimbra that you use to archive all inbound and outbound email and want to use that account to copy the emails into RedHorse. I assume you are not using Outlook as your client and can not use RHOLink to do the copying of the emails.
First, any emails brought into RedHorse through the Zimbra account will be seen by RedHorse as inbound emails, even if they were originally outbound emails in Zimbra. It may be confusing to the users when all emails look like they were retrieved when they were actually sent.
The email rule you are using to link emails to the account is not necessary. If the "FromAddress" exists in RedHorse, it will be automatically linked without an email rule. Keep in mind that the emails received in Zimbra will be linked to the customers record (the FromAddress is the customer) but that emails sent from Zimbra will be linked in RedHorse to the users contact record (the FromAddress is your user). Your customer record will be incomplete as far as email history.
All emails brought in from Zimbra will be linked to the userid you created to do the email retrieval. They will not be linked to each user in RedHorse.
The function to create new contacts from unlinked emails is done by the user when reading the email. The user can select any of the recipient addresses and use it to link to an existing contact or create a new one. A function to automatically create contacts could potentially create new contact records for every spam email received by the email center.
The best method to achieve what you are trying to do is to use RHOLink if your users are able to do their emails in Outlook.