Is it the usual practice in your discipline for the postdoc in the group to be placed as co-author as well as a corresponding author on all publications that are written by the PhD students in the group?
For some publications, the postdoc provides input and advice, and for others none whatsoever. The reasoning behind this practice is that the postdoc needs publications for his academic career. However, the PhD students that do 95% of the work in some cases, are the only ones able to answer scientific questions. As a consequence, emails are prepared for the postdoc by the PhD students, and sent out under the name of the postdoc.
Is this a standard practice in your field of research?