code: public Integer postCharactersCharacterIdMail(Integer characterId, String datasource, String token, Mail mail)
              throws ApiException {
          ApiResponse<Integer> resp = postCharactersCharacterIdMailWithHttpInfo(characterId, datasource, token, mail);
          return resp.getData();
      }
docs: Send a new mail Create and send a new mail --- SSO Scope:
  esi-mail.send_mail.v1
  
  @param characterId
  An EVE character ID (required)
  @param datasource
  The server name you would like data from (optional, default to
  tranquility)
  @param token
  Access token to use if unable to set a header (optional)
  @param mail
  (optional)
  @return Integer
  @throws ApiException
  If fail to call the API, e.g. server error or cannot
  deserialize the response body