Mailbox admin and admin email

I made a mailbox with an email address on my forums. However, when I try to change over emails when sending out newsletters, it says my email is not valid. Anyone help?

I go into admin>general>newsletters>write and send> and when I want to change it, it says it isn't valid. I wanted to use this email instead of my private one as my admin email.

Has this worked for anyone? Am I doing something wrong? I don't know what I can be doing wrong.