39 CFR §3024.8

1. (a)As an affirmative defense to an allegation of a violation of 39 U.S.C. 404a(a), the Postal Service may demonstrate that it is specifically authorized by law to take the action or inaction alleged to be a violation of that section.
2. (b)Authority under 39 U.S.C. 401 or 39 U.S.C. 404 may not form the basis of an affirmative defense under paragraph (a) of this section.
3. (c)Paragraph (b) of this section does not preclude the Postal Service from arguing that a particular Postal Service regulation or other action (or inaction) does not have the requisite effect to violate 39 U.S.C. 404a(a).