Learn German: Die E-Mail blieb unbeantwortet, worüber der Chef wütend war. - The email remained unanswered, which the boss was angry about.

Using bleiben with an un- prefixed past participle emphasizes a continuous state of non-occurrence. While the passive wurde nicht beantwortet simply states that the action didn't happen, blieb unbeantwortet highlights that the email sat there in a lasting "unanswered" condition.

Why is worüber used here instead of a relative pronoun like das?

worüber is a wo-compound acting as a relative pronoun. We use wo-compounds when referring back to an entire preceding clause or idea, rather than a specific noun. Here, the boss isn't angry at the email itself, but at the entire situation of it remaining unanswered.

Where does the über part of worüber come from?

It is triggered by the adjective wütend. In German, you express being angry about something using the combination wütend über. Since the relative pronoun must include the preposition associated with the emotion, we combine wo + r + über.

Since unbeantwortet describes the feminine noun E-Mail, why doesn't it have an adjective ending?