A Reisevollmacht is a document confirming that a minor may travel with an adult who is not the legal guardian. As Roddy explained in his comment, an email cannot possibly replace a Reisevollmacht if you should really need one. It might be helpful to convince the airline that no Reisevollmacht is necessary in your case.
If the email explains why no Reisevollmacht is necessary in your case, with a reference to the applicable laws, then having it along should help the airline staff to look things up, as explained by Relaxed. If it simply says "not necessary" without further explanation, it is useless for your purposes.
Also, an airline may have internal guidelines which go beyond those required by law, and make them part of the fine print in your contract. Then the email won't help, either.