The Illinois Department of Revenue (IDOR) sends letters and notices to request additional information and support for information you report on your tax return. You can find the CP or LTR number on the right corner of the letter.You must file Form IL-1040-X, Amended Individual. Income Tax Return, if. Access IRS forms, instructions and publications in electronic and print media. A tax exempt organization may need a letter to confirm its taxexempt status or to reflect a change in its name or address. Complete lines 1a-4 on the top portion of the form. An IRS Verification of Non-filing Letter provides proof that the IRS has no record of a filed Form 1040, 1040A or 1040EZ for the year you requested. It may take up to 75 calendar days for us to process your request. Attach IRS form 12661.