Learn Portuguese (Portugal): Quem respondeu ao email dele?

It is the simple past tense of the regular verb responder. For regular -er verbs, the 'he/she/it' ending in the past tense is -eu. We use this form because the question word quem (who) acts as a third-person singular subject.

Why do we say ao email instead of just o email?

The verb responder (to reply or answer) requires the preposition a (to) before the thing or person receiving the reply. Since email is a masculine noun, the preposition a combines with the masculine word for 'the' (o) to form the contraction ao (to the).

I thought 'his' was seu. Why does this sentence use dele?

While seu can mean 'his', it can also mean 'hers' or 'yours', which often causes confusion. To be perfectly clear, Portuguese speakers usually put dele (a combination of de and ele, literally 'of him') after the noun instead. So, o email dele translates literally as 'the email of him'.

Does the word quem always take this verb form?

Yes! Just like in English where we say 'Who is' rather than 'Who are', the question word quem (who) always takes the third-person singular form of the verb.