docs: def is_full_mxid(user_string):
      """Returns True if a string is a valid mxid."""
      if not user_string[0] == "@":
          return False
      parts = user_string[1:].split(":")
      localpart_chars = ascii_lowercase + digits + "._-="
      if not (len(parts) == 2 and all([i in localpart_chars for i in parts[0]])):
          return False
      return True