Learn Portuguese (Portugal): Ela abriu o email, mas não respondeu.

Abrir (to open) is a regular -ir verb. To talk about a completed action she did in the past, you drop the -ir ending and add -iu. This makes abriu (she opened).

Does respondeu follow a similar rule since it is an -er verb?

Yes! The verb is responder (to reply). For regular -er verbs talking about 'ele', 'ela', or 'você' in the simple past tense, you drop the -er and add -eu. That gives us respondeu (she replied).

Why isn't the word ela repeated after mas?

In Portuguese, you can leave out the subject pronoun if it is already clear who you are talking about. Since the first half of the sentence says that she opened the email, it is obvious that she is also the one who did not reply.

Is the word email masculine or feminine in Portuguese?

It is masculine, which is why we use the masculine word for 'the' before it, giving us o email. When modern English words are borrowed into Portuguese, they almost always become masculine nouns.