Zoho won't let me add a different sender - says it's a duplicated email

Zoho won't let me add a different sender - says it's a duplicated email

I'm trying to set up an email campaign and need to add a different sender/reply to email than my login. It says the email I'm adding is a duplicate. How can I use that particular email? It's not in my drop down menu to select senders and I can't see where else it would be duplicated.