Just like you receive a W2 or 1099 form at the end of the year from employers and the IRS gets a copy of that form, your account custodian will file a Form 5498, IRA Contribution Information where you get a copy and the IRS gets a copy.The information on the forms goes into the IRS computer and matches up what is on the 5498 to what you report on your tax return. If the information on the two forms does not agree, the assumption is your tax return is incorrect, so you get a match up letter asking for more details and support for what you put on your return.