When I post on Stack Overflow a configuration file from a Linux distribution that I have modified, can I let the part where the original author is mentioned untouched or do I have to remove the original authors name/email-address?
It is usually not a bad idea to remove author information - it is not pertinent to the question.
You often want to post only the relevant parts of a file anyway.
In some cases however, technically, the license a file is published under might require you to leave the author info intact. It could also be that the license makes it entirely impossible to post the file on SO because it's incompatible with SO's CC-Wiki license.
You'll have to use your own judgement in these cases.