They will need more information to go on to recover your old id.
Have you got any DRM (digital rightes management) books already on the Kobe or elsewhere that you still want to be able to read,
or are you only interested in the ability to read new books?
If you only want to read new books, just get yourself a new Adobe ID and forget all about the old one.
Register all your devices with that new ID. Query back if you need help with that.
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If you want to read old books,
If you search about on your Kobo there may well be a settings or help or information page that will tell you your adobe ID the Kobo is registered with.
I haven't got a Kobo so can't help with details there.
Or Adobe should be able to help if you give them your Adobe userid in its 'internal' format (urn:uuid:)
I can tell you how to get at that but it is a little complicated.
So first check with Live Chat and ask them if they can tell you the email address if you give them the urn:uuid:
or indeed if there is any other way they can recover it.
Adobe Live Chat: http://www.adobe.com/support/chat/ivrchat.html
If they can, then I'll tell you how to get at that internal id.