Learn Farsi: نامه را خدمت جناب مدیر فرستادم. - I sent the letter to the honorable manager.

In formal or respectful Farsi (part of the Ta'arof system), khedmat-e (literally "to the service of") replaces be when you are directing an action or giving something to a highly respected person. It elevates the manager's status.

Does jenâb require an Ezafe when placed before a title like modir?

Yes. When using an honorific like jenâb (meaning Honorable or Excellency) before a title or a surname, you must link them with an Ezafe. Here, it is pronounced jenâb-e modir.

Since this lesson is about using plural forms to show respect, why is the verb ferestâdam (I sent) in the singular?

Plural forms for respect (like using shomâ for one person, or a 3rd-person plural verb for a boss) only apply to the person you are showing respect to. In this sentence, you are doing the sending. You never use respectful plural forms for yourself! The respect here is entirely shown through the vocabulary (khedmat-e and jenâb-e).

Just to review, why is râ strictly necessary after nâmeh here?

The word râ marks a specific, definite direct object. Because you sent a specific letter (the letter) to the manager, rather than just any letter, râ is required. Without it, the sentence would mean "I sent a letter."