Meaning of 'authenticity'

Question

As I am dubious of your authenticity here (a mailing list), I cannot spend any time with you

Is he dubious that I'm not a real user or I'm not a expert?

Answer 1

With no further context I would say that the writer has doubts about your motivation for asking for assistance. If he believed that your question was an "authentic" question, where you had a genuine curiosity and a valid need to get a good answer, he would be willing to spend his time giving you that answer.
However, for some reason, he thinks that your motive for asking may be something else entirely: it could be to compare the answer you get there to what you already know, or to an answer you got somewhere else, or just to see how quickly you get an answer, or some other factor; the answer itself is not what you're interested in, it's more likely that you're trying to find out something about the answering process. Therefore, he's not willing to spend time giving you an answer.

Answer 2

He is saying that he thinks that either...

(1) you are not the person that you are saying you are

or

(2) what you said may be somebody else's words.

Answer 3

Without any more context it is impossible to tell whether his doubt is related to your expertise. Within the scope of the quote, I would guess he doesn't think you are a registered (or otherwise vetted) user of the mailing list.