Learn Dutch: Het citaat ontbreekt, dus wij sturen hun de e-mail. - The quote is missing, so we send them the e-mail.

In formal written Dutch, hun is used for indirect objects (the receiver of the action) when there is no preposition in front of it. Here, hun is the indirect object receiving the email. If we rephrased the sentence to include the preposition aan (to), we would have to switch to hen: wij sturen de e-mail aan hen.

Would native speakers actually say "wij sturen hun" in everyday conversation?

Probably not. While you need to know this strict hun vs hen rule for formal writing, it is mostly ignored in spoken Dutch. In everyday conversation, native speakers almost always use the unstressed pronoun ze (them) instead. You would be much more likely to hear: wij sturen ze de e-mail.

Why doesn't the word order invert after "dus"? Shouldn't it be "dus sturen wij"?

Dus (so/therefore) acts as a coordinating conjunction here, meaning it joins two main clauses without affecting the word order. Just like with en (and) or maar (but), the subject and verb in the second clause stay in their normal order: wij sturen.

How exactly does the verb "ontbreken" work in this sentence?

Ontbreken means "to be missing" or "to be lacking". In Dutch, the thing that is missing or lacking is the subject of the sentence, not the direct object. That is why het citaat (the quote) is the subject here, giving us the literal translation "The quote is missing."