I am new to zohomail and I thought that what I am trying to accomplish would be a simple configuration change but it seems that I cannot get it to work.

This is what I'm trying to do:

I build websites for small businesses and I would like to set my client up with her own info@herdomain.com email address.

Because I have several small business clients I thought I could manage many different domain emails under my zoho account and then give full access to the specific clients just by associating a user to each specific email address.

This is one of the reasons I chose zohomail.

The exact situation:

I bought a standard domain specific email ($24/year) : info@vizzio.com

Now my default mail account is info@vizzio.com.

I added an external user to my zoho organization (to the vizzio.com domain).

Now I would like to give that external user full access to the info@vizzio.com.

BUT The only thing I can do is create a new email for that external user. (and that is not what I want to do)

I would like to simply associate that external user to what is currently my default email address. (the info@vizzio.com address) I want to transfer ownership, or at least total access, privileges to that email account.

Steps I already attempted since I can't seem to associate the info@vizzio.com with that external user account: 

I was thinking to allow the client to use her gmail to access the info@vizzio.com address but that doesn't work because of an "authentication error" I am 100% certain that the user name and password are correct so it's not my fault that I cannot get her gmail to access her info@vizzio.com address (using my credentials, my authentication information)..  It seems to me that because gmail and zoho are competitors they don't want to play nice.

Thanks.