One reason could be because you have already used this email before and deleted the user, but the user meta has not been deleted, so when you try and use that email again it throws up an argument saying email already in use and due to this; you will get a does not exist page when trying to use this mail or user account at login.
So the problem you have, is you need to delete that data in the database. relating to the user and the i think its a 32 digit hexidecimal key??????
although simple you just delete the user if you are unsure about what you are doing then this would be an unwise move if that user is the owner, only admin or super admin
There are user meta plugins that do this for you but I have never used one so I would look for the popular ones and read what they can do
There are also exporters and importers to and from cvs that do the same but again I dont know if they work.
Please back up your DB and please read the instructions in this plugin
if you have access to php my admin or mysql then the easy way is to save your database to your hard drive, open it in something like notepad++ do a search for all instances of that email address and delete them, save this as a different named sql file and re upload it to your sites database. if you recieve an error it could be due to you trying to insert tables into tables that already exist, so you have to delete the entire database (Drop) and then upload the second version again.
only then will you be able to use that fred@fred email again.
I think that is correct!
