Since 2013 (tax year 2012), a CRA mandate has required that tax preparers who file more than 10 returns must file them electronically via EFILE.
This mandatory electronic filing applies to both T1 General income tax and benefit returns and T2 corporation income tax returns for 2012 and later tax years.
For exceptions, EFILE registration information, and other information about mandatory electronic filing for tax preparers, review the CRA article Mandatory electronic filing for tax preparers.
For more details about filing a return electronically, review the CRA's Frequently asked questions article.