Serienummeret og svarfristen står i emailen.

Because Danish very often makes compound nouns by joining words together.

• serienummer = serial number
  • serie
    • nummer
• svarfrist = reply deadline / response deadline
  • svar
    • frist

This is extremely normal in Danish. Where English often writes two words, Danish often uses one.

They mark the nouns as definite, so they mean the.

• serienummer = a serial number

• serienummeret = the serial number

• svarfrist = a reply deadline
• svarfristen = the reply deadline

So Danish usually adds the to the end of the noun instead of putting a separate word before it.

Because Danish often uses a suffix article instead of a separate article.

In English:

• the serial number
• the response deadline

In Danish:

• serienummeret
• svarfristen

This is one of the biggest structural differences between English and Danish nouns.

Because Danish nouns belong to different grammatical genders.

• et serienummer → serienummeret
• en svarfrist → svarfristen

Very roughly:

• common gender nouns usually take -en in the definite singular
• neuter nouns usually take -et in the definite singular

So the ending depends on whether the noun takes en or et in its basic form.

In Danish, stå is often used when information is written somewhere.

So står i emailen means something like:

• is written in the email
• appears in the email
• is stated in the email

This is more idiomatic than just using er here.
English usually says is in the email, but Danish often prefers står when talking about written text.

Because Danish verbs do not change form for singular vs. plural subjects in the present tense.

So you get:

• Serienummeret står ...
• Svarfristen står ...
• Serienummeret og svarfristen står ...

The verb stays står in all of these.

i usually means in.

So i emailen means in the email.

It is the natural preposition here because the information is understood as being inside the text of the email.

It is widely understood, but in more standard writing many people would prefer e-mailen or simply mailen.

So:

• emailen = common in everyday writing
• e-mailen = more traditional/standard-looking
• mailen = also very common

A learner should recognize all of them.

Yes. Danish often moves something to the front of the sentence for emphasis, but then the verb still stays in second position.

So you can also say:

I emailen står serienummeret og svarfristen.

This means the same thing, but puts more focus on where the information is.

The basic forms are:

• serienummeret → et serienummer
• svarfristen → en svarfrist
• står → at stå
• emailen → en email / often en e-mail or en mail

This is useful because Danish dictionaries usually list nouns in their indefinite singular form and verbs in the infinitive.