According to his website, " if your query is about benefits or pensions or Iain’s role as Secretary of State for Work and Pensions, please write directly to the Department for Work and Pensions at:
The Rt Hon Iain Duncan Smith MP
Secretary of State, Department for Work and Pensions,
Caxton House, Tothill Street, London, SW1H 9DA
Or email: firstname.lastname@example.org"
Whether he takes any notice is another matter, but I'll give it a try.