code: public void putCharactersCharacterIdMailMailId(Integer characterId, Integer mailId, String datasource,
              String token, MailMetaData mailMetaData) throws ApiException {
          putCharactersCharacterIdMailMailIdWithHttpInfo(characterId, mailId, datasource, token, mailMetaData);
      }
docs: Update metadata about a mail Update metadata about a mail --- SSO Scope:
  esi-mail.organize_mail.v1
  
  @param characterId
  An EVE character ID (required)
  @param mailId
  An EVE mail 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 mailMetaData
  (optional)
  @throws ApiException
  If fail to call the API, e.g. server error or cannot
  deserialize the response body