Learn Hardcore German: Bitte senden Sie mir die Zugangsdaten erneut, falls die E‑Mail nicht ankommt. - Please send me the login credentials again if the email does not arrive.

erneut: neutral-formal “again/anew”; good in professional writing.
noch einmal/nochmal: “one more time,” neutral to colloquial.
wieder: general “again,” fine here but a bit less formal than erneut .

The sentence uses the formal form of address Sie (capitalized), appropriate for professional or polite contexts. It addresses someone you don’t know well, a customer, support agent, etc. Using du would be informal and only for people you know well.

Is “senden Sie …” an imperative? How is the formal imperative formed?

Yes. The formal imperative uses the infinitive + Sie: Senden Sie …. With du, it’s Sende … (often colloquially “Schick …”), and with ihr, Sendet ….

Why is it mir and not mich?

Because senden takes a recipient in the dative and a thing in the accusative: senden (jemandem – dative) (etwas – accusative). So: senden Sie mir (DAT) die Zugangsdaten (ACC). Mich (accusative) would mean “send me (somewhere)”—a different meaning.

Which case is die Zugangsdaten in, and why?

It’s the direct object in the accusative plural. In the plural, the article is die in both nominative and accusative, so it looks the same. Dative plural would be den Zugangsdaten.

Why does ankommt go to the end after falls?