What does the Cook Co. Recorder deem to be a recordable document?
All property related documents must contain a Property Identification Number (PIN), a complete legal description (available from the County Clerk's office or prior documents on that parcel) and a commonly known address for the property. The grantee's name must appear on the face of all deeds and a return mail address must appear on the face of the document.
Also, documents affecting title must have a preparer's name, a Real Estate Tax Declaration form (municipal, county, state where applicable) or exemption forms
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No one can know what the record is in the case because online we cannot find out any details. Check with a lawyer in your locale to discuss more of the details.
Unfortunately, to be valid in Illinois, an assignment does not need to be recorded - Recording is just "notice" and does not affect validity. An assignment can be done verbally or electronically without the need to memorialize with the Recorder's Office that it has been done. The real issues regarding the validity of assignments arise when there is a foreclosure or if there is a dispute regarding the correct accounting of your payments to the correct holder of your Note and Mortgage.
Notarization, too, does not make a document valid or invalid for recording. It is a statement that the signature was actually signed in front of someone who acknowledged that they checked on the identity of the person signing - which might be significant for a court hearing, but not necessary to validate the document.
You should read the document carefully though - a mortgage cannot be assigned without the underlying debt, i.e. the Note. You may want to make a Qualified Written Request of your lender to prove that they have the property documents to support your obligation to pay the entity you are paying.
As to what the Recorder deems to be a recordable document - they basically will take anything given to them to record - although certain documents, such as deeds, require certain formalities to be accepted for recording. If the Recorder's employees are not sure of the validity of the document sought to be recorded, they will often stamp "Recorded at Customer's Request" at the bottom, implying that they are not passing on the validity of the document.
As an aside, if you are looking at this because of an impending foreclosure, there may or may not be a valid claim that the assignment is not effective. However, by what you have stated, you have not gotten to the issue the Court will look at. You should locate effective foreclosure defense counsel.
This answer does not constitute legal advice and does not and is not intended to create an attorney-client relationship. The law may vary depending on the state in which you reside. It is intended only to give some direction in which to seek assistance.