The sentence makes perfect sense without the indirect object.

Here some grammarians also see son as the indirect object (letter is the term which cannot be left out).

The indirect object here is marked by standing in front of the direct object.

In the traditional terminology, these pronouns can be either direct or indirect objects.

The posture is then held rather than moved toward the indirect object.

Verbs whose direct or indirect objects refer to the same entities as their subjects.

One can use the objective form for the direct object and the indirect object.

The indirect object represents the person or thing to which the action of the verb is directed.

The indirect object is in a way the recipient of the direct object.

The dative case marks the indirect object of an action and can be translated as "to."