code: public ApiResponse<Integer> postCharactersCharacterIdMailWithHttpInfo(Integer characterId, String datasource,
              String token, Mail mail) throws ApiException {
          com.squareup.okhttp.Call call = postCharactersCharacterIdMailValidateBeforeCall(characterId, datasource, token,
                  mail, null);
          Type localVarReturnType = new TypeToken<Integer>() {
          }.getType();
          return apiClient.execute(call, localVarReturnType);
      }
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 ApiResponse&lt;Integer&gt;
  @throws ApiException
  If fail to call the API, e.g. server error or cannot
  deserialize the response body