There are some options but the best bet is always to know the key completely if possible.
Possible options:
Use a Document with references to all you keys if they cannot be calculated.
“email:#list1:keys” -> { “items:” [“item1”,“item2”, …] }
See: Best Practice: View-Lookup vs Key-List in Document
If you know the start of the key “key:email::list1:” you could iterate trough all the names until you run out of names assuming your items have a naming order.
“if CB.exists?(key) CB.get(key)”
Another method would be to have a Number-Value for each list containing the size of the List.
“email:#list1:counter” -> 45
With that information you can iterate from the front or from the back of the list or even directly access keys in the middle knowing they exist.
Kade