For MueLu bug reporting:

https://github.com/trilinos/Trilinos/issues (preferred)

muelu-developers AT software DOT sandia DOT gov

For MueLu help (requires membership):

muelu-users AT software DOT sandia DOT gov or

trilinos-users AT software DOT sandia DOT gov

For general Trilinos questions:

trilinos-users AT software DOT sandia DOT gov