Problem with sender email

I am having an issue when I try and select another email address to send a campaign from in Zoho Campaigns. I tried to "Add Sender's Address" and when I add the address I want the campaign to be from, it says "Duplicated email address found while adding your sender address." Yet it is not listed on the drop down. I ran a general search for the email address in Zoho Campaigns and it didn't show up anywhere so I am not sure where it is....Can you help me?