Learn Dutch: De enquête wordt naar alle respondenten gestuurd.

In Dutch, the passive voice for an ongoing or regular action uses the verb worden (to become), not zijn (to be). If you said De enquête is gestuurd, it would translate to "The survey has been sent" (meaning the action is already finished). Because the survey is being sent as a current or general process, we use wordt.

Could I use the preposition aan instead of naar here?

Yes, you could easily say aan alle respondenten gestuurd. Both sturen naar (send to/towards) and sturen aan (send to the attention of) are correct. Naar emphasizes the destination or movement, while aan focuses slightly more on the recipient, but in this context they are completely interchangeable.

Why is it alle respondenten and not just al respondenten or allemaal respondenten?

The word al gets an '-e' ending to become alle when placed directly before a plural noun without an article to mean "all". You would only use al if a definite article was present (like al de respondenten). Allemaal is an adverb used elsewhere in the sentence, rather than acting as an adjective directly in front of the noun.

The word enquête has a circumflex (the little hat) on the 'e'. Is this standard Dutch spelling?