'unable to verify because its already in use' comment

in iMessage i'm trying to add an email and i keep getting this 'unable to verify' comment which is frustrating because the email is not associated with another apple id as i've checked iCloud,iMessage,Game Center,app centre,iTunes ...its not associated with any of these. i took it off one that it used to be associated with but it doesn't seem to matter as i still cannot add the email address into iMessage. I'm trying to retrieve some deleted text messages and i need to add this account back in order to do this.