Learn Dutch: Ik mailde hem omdat de website offline was. - I emailed him because the website was offline.

Even though mailen is an English loanword, it follows standard Dutch conjugation rules. To form the past tense, we take the stem mail. The final letter is 'l'. Because 'l' is not a voiceless consonant (meaning it is not in the mnemonic 't kofschip or soft ketchup), we add -de instead of -te.

Is it natural to use the English word offline in Dutch?

Yes, it is completely standard. Dutch has fully adopted offline and online for internet and technology contexts. While you could technically describe the situation using Dutch phrases like niet verbonden or uit de lucht (off the air), using the loanword offline is by far the most natural and common choice.

Why does the verb was jump to the very end of the sentence?

This happens because of the word omdat (because). It is a subordinating conjunction, which introduces a dependent clause. As a refresher: in Dutch dependent clauses, all verbs are pushed to the very end. That is why was comes after offline.

How would the word order change if I used want instead of omdat?