Is quantitative easing part of the ECB's and Fed's mandates or is it an exceptional policy that they used, especially in the financial crisis 2007-2008?
It’s a means to an end, and to the mandate that these institutions imposed on themselves. We could also call it an “unofficial” mandate since politicians would likely put pressure on them in indirect ways to uphold their primary mandate. That primary mandate is a 2% inflation target. QE of course expands the money supply which can expand the velocity of money and keep prices relatively stable, as we already know. So QE is a means to that mandate end so that prices don’t fall sharply as we were beginning to see following the financial crisis. QE in it of itself is not a mandate.