I can't add a new email account it says it already exists, but it's impossible.

I can't add a new email account it says it already exists, but it's impossible.

I'm trying to add a new email account security@shoshonepharmacy.com, but it says already exists which is impossible. I had to do it as shoshonesecurity@shoshonepharmacy.com how do I change it when it won't let me?!