Das Rücksendeetikett ist in der E-Mail.

Because Rücksendeetikett is a neuter noun in German, so its nominative singular article is das.

• das Rücksendeetikett = the return label

In this sentence, Rücksendeetikett is the subject, so it stays in the nominative case:

• Das Rücksendeetikett ist ...

A good thing to remember is that the grammatical gender of a noun in German does not always match any logic from English, so you usually have to learn the noun and its article together:

• das Etikett
• therefore also das Rücksendeetikett

It is a compound noun, which is very common in German.

It can be understood as:

• Rücksendung = return / sending back
• Etikett = label

So Rücksendeetikett means return label.

In compounds, German often combines ideas into one long word instead of using separate words like English does. The last part usually determines the grammatical gender. Here, the last part is Etikett, which is neuter, so the whole compound is neuter:

• das Etikett
• das Rücksendeetikett

Because all nouns are capitalized in German.

So in this sentence:

• das Rücksendeetikett
• die E-Mail

both nouns begin with capital letters.

This is one of the most noticeable spelling differences from English.

Because in can take either the dative or the accusative, depending on the meaning.

Here, in der E-Mail means inside the email or contained in the email, which describes a location, not movement. For location, German uses the dative after in.

• die E-Mail = nominative
• der E-Mail = dative

So:

• Das Rücksendeetikett ist in der E-Mail. = The return label is in the email.

If there were movement into something, German would use the accusative:

• Ich lege das Dokument in die E-Mail. = I put the document into the email.

Because E-Mail is a feminine noun, and feminine nouns change article in the dative singular:

• nominative: die E-Mail
• accusative: die E-Mail
• dative: der E-Mail
• genitive: der E-Mail

Since in here expresses location, it requires the dative, so die E-Mail becomes der E-Mail.

German main clauses usually follow the verb-second rule, often called V2 word order. That means the finite verb normally comes in the second position of the clause.

Here the sentence is:

• Das Rücksendeetikett | ist | in der E-Mail

So:

1. Das Rücksendeetikett
2. ist

This is very normal German word order:

• subject first
• verb second
• other information after that

You could also move another element to the front, but the verb would still stay second:

• In der E-Mail ist das Rücksendeetikett.

That version is less neutral, but grammatically correct.

Yes, you could, and it is grammatically correct. But it sounds a bit more formal or heavier than necessary in everyday language.

Compare:

• Das Rücksendeetikett ist in der E-Mail. = simple, natural
• Das Rücksendeetikett befindet sich in der E-Mail. = more formal, more like is located in

In normal customer-service or everyday contexts, ist in der E-Mail is very natural.

Yes, it is natural, especially in practical contexts like customer service, online shopping, or instructions.

Depending on context, Germans might also say:

• Das Rücksendeetikett finden Sie in der E-Mail.
  = You can find the return label in the email.
• Das Rücksendeetikett ist in Ihrer E-Mail enthalten.
  = The return label is included in your email.
• Das Rücksendeetikett wurde Ihnen per E-Mail geschickt.
  = The return label was sent to you by email.

But Das Rücksendeetikett ist in der E-Mail is perfectly understandable and natural.

A rough English-friendly guide would be:

• Rück ≈ like ruek with a short rounded vowel
• sende ≈ ZEN-duh
• etikett ≈ eh-tee-KETT

Very roughly:

• RUEK-zen-duh-eh-tee-KETT

A few pronunciation notes:

• ü in Rück- does not exist exactly in English. It is a rounded front vowel.
• s in sende sounds like z
• the stress in Etikett falls on the last syllable: etiKETT

The standard form is E-Mail with a hyphen.

You may sometimes see:

• Email

but in standard German, especially in careful writing, E-Mail is preferred.

Also remember its gender:

• die E-Mail

Example forms:

• die E-Mail
• in der E-Mail
• per E-Mail

It is in the nominative case because it is the subject of the sentence.

Ask yourself: what is in the email?

Answer:

• Das Rücksendeetikett

That makes it the subject, so German uses the nominative:

• das Rücksendeetikett

Meanwhile, in der E-Mail is a prepositional phrase in the dative because it expresses location.

So the sentence contains:

• nominative subject: das Rücksendeetikett
• dative phrase: in der E-Mail