Is it ethical for the postdoc to be listed as a co-author and the corresponding author on all group publications? 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?
I come from a field that is rather (let's call it) pragmatic about co-authorship, and I have never heard of a postdoc automatically being a co-author of papers. Sure, sometimes the input of the postdoc is not extremely large, but having a postdoc as a default co-author of each paper irrespective of contribution seems very uncommon to me.


  The reasoning behind this practice is that the postdoc needs publications for his academic career. 


Who cares? The same is true for you. If he cannot write papers himself or actually advise students that write good papers (not the same as just being on a paper without contribution), he should not be able to progress his career. 


  As a consequence, emails are prepared for the postdoc by the PhD students, and sent out under the name of the postdoc.


That's ridiculous. Don't do that (either for a postdoc or a professor). If they are not able to answer questions about a paper, they need to at least be honest enough to defer to the person that can. Pretending like the postdoc is the real brain behind a paper is certainly not ethical.


  Is this a standard practice in your field of research?


Nope. Never heard of that, and Applied CS is certainly not a field that is well-known for its die-hard adherence to ethical guidelines re: co-authorship.