docs: public com.squareup.okhttp.Call getCharactersCharacterIdMailMailIdAsync(Integer characterId, Integer mailId,
              String datasource, String ifNoneMatch, String token, final ApiCallback<MailResponse> callback)
              throws ApiException {
  
          com.squareup.okhttp.Call call = getCharactersCharacterIdMailMailIdValidateBeforeCall(characterId, mailId,
                  datasource, ifNoneMatch, token, callback);
          Type localVarReturnType = new TypeToken<MailResponse>() {
          }.getType();
          apiClient.executeAsync(call, localVarReturnType, callback);
          return call;
      }