S-Corporations that file 10 or more information returns — including W-2s, 1099s, and other required filings — in the aggregate during the calendar year must e-file their federal 1120-S. This threshold dropped from 250 to 10 under final Treasury regulations that took effect for returns filed after January 1, 2024. Consequently, most active S-Corps now fall under the mandatory e-file requirement. Entities below the threshold may still e-file voluntarily — which the IRS actively encourages.