Sha256 is a hash function. It will create a hash value which is unique for the particular input. But it is almost impossible to do the reverse (get the input from the hash). You can try the "crypString()" function. It will encrypt the input with the default key (if no key is specified). Then you can use decryptString() to get back your input.
(here with nms:subscription addressSpecific field, but of course you can do it with nms:recipient.email field).
Please see below the Query expression editor/advanced choice: