code: public List<MailHeadersResponse> getCharactersCharacterIdMail(Integer characterId, String datasource,
              String ifNoneMatch, List<Integer> labels, Integer lastMailId, String token) throws ApiException {
          ApiResponse<List<MailHeadersResponse>> resp = getCharactersCharacterIdMailWithHttpInfo(characterId, datasource,
                  ifNoneMatch, labels, lastMailId, token);
          return resp.getData();
      }
docs: Return mail headers Return the 50 most recent mail headers belonging to
  the character that match the query criteria. Queries can be filtered by
  label, and last_mail_id can be used to paginate backwards --- This route
  is cached for up to 30 seconds SSO Scope: esi-mail.read_mail.v1
  
  @param characterId
  An EVE character ID (required)
  @param datasource
  The server name you would like data from (optional, default to
  tranquility)
  @param ifNoneMatch
  ETag from a previous request. A 304 will be returned if this
  matches the current ETag (optional)
  @param labels
  Fetch only mails that match one or more of the given labels
  (optional)
  @param lastMailId
  List only mail with an ID lower than the given ID, if present
  (optional)
  @param token
  Access token to use if unable to set a header (optional)
  @return List&lt;MailHeadersResponse&gt;
  @throws ApiException
  If fail to call the API, e.g. server error or cannot
  deserialize the response body