Domtar (UFS) 10-Q 2018 Q1 Quarterly report

Exhibit 32.1

CERTIFICATION BY THE CHIEF EXECUTIVE OFFICER

PURSUANT TO 18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT

TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

The undersigned hereby certifies that to his knowledge, the Company’s Quarterly Report on Form 10-Q for the period ended March 31, 2018 (the “Form 10-Q”) fully complies with the requirements of section 13(a) or 15(d) of the Securities Exchange Act of 1934 and information contained in the Form 10-Q fairly presents, in all material respects, the financial condition and results of operations of the Company.

Date: May 9, 2018

/s/ John D. Williams

John D. Williams

President and Chief Executive Officer