An Email cannot be use for two users. There are a couple of options you can use to fix this, but first, you need to determine where that email is already in use.

   Go up to the search bar and search for the email address in question.
   On the search results page, click the Users tab.
   Click on the user/email, this will bring you over to the profile page (alternatively, click the Admin icon (), select Manage > People, and search for the email in question).
   From there, you have two options:

           Update the user indicated in the error message. Change their email address on file to something else - you will need it to be a valid email address they can receive an email at.
           Delete the user. If you just created the user for testing purposes and don't need an agent to log in with that email address, simply delete them. Instructions for that can be found here!

Once you've freed up the email address, you can go back into your email settings and add it as a support address.

Hi Aki_66 --

Thanks for your reply. Unfortunately I don't understand where you're asking me to go for these settings. I use DuckDuck and if I enter my intended secondary email address (the one that's being rejected) in the search bar for the browser like you're suggesting, nothing comes up for direct match. I'm lost from there. Can you point me in the right direction? Thanks