The Letter Audit (or Correspondence Audit) is conducted entirely by mail. The IRS will send a letter asking for additional information about an item on your return, such as a deduction claimed for charitable contributions or medical expenses. This type of audit could become an office audit if the IRS is not satisfied with your reply.

Letter audits often notify the taxpayer of mathematical or clerical errors made on the return, or a failure to report income that has been reported to the IRS by an employer or other payer.