Det är först när mottagaren läser ämnesraden noggrant som hon förstår vad mejlet gäller.

This is an emphatic Swedish pattern meaning it is only when ... that ... or not until ... does ....

So:

• Det är först när mottagaren läser ämnesraden noggrant som hon förstår ...
• = It is only when the recipient reads the subject line carefully that she understands ...

Here först does not mean firstly. It means something more like only then / not until then.

In this structure, som links the emphasized part to the main statement.

Pattern:

• Det är först när X som Y
• = It is only when X that Y

So in your sentence:

• först när mottagaren läser ämnesraden noggrant = the emphasized condition
• som hon förstår vad mejlet gäller = what happens under that condition

You should think of som here as part of the fixed pattern, not as the normal relative pronoun who/that/which.

Not really in a concrete way. This det is a formal or introductory det, often used in Swedish in cleft-like sentences for emphasis.

Compare:

• Mottagaren förstår vad mejlet gäller först när hon läser ämnesraden noggrant.
• Det är först när mottagaren läser ämnesraden noggrant som hon förstår vad mejlet gäller.

Both can express roughly the same idea, but the second one is more focused and emphatic. The det helps build that structure.

Because när mottagaren läser ämnesraden noggrant is a subordinate clause.

In Swedish, subordinate clauses normally keep the order:

• subject + verb

So:

• mottagaren läser
• not läser mottagaren

This is different from many main clauses in Swedish, where the finite verb often comes in second position.

Because they mean the recipient and the subject line, not just a recipient or a subject line.

• mottagare = recipient

• mottagaren = the recipient

• ämnesrad = subject line
• ämnesraden = the subject line

Swedish usually adds definiteness at the end of the noun:

• -en or -et, depending on the noun

Here, the sentence is talking about a specific recipient and a specific subject line, so the definite form is natural.

Ämnesrad means subject line, especially in an email.

It is made from:

• ämne = subject/topic
• rad = line

So ämnesraden literally means the subject line.

This is a very common word in email-related Swedish.

Because noggrant is an adverb, while noggrann is an adjective.

• noggrann = careful
• noggrant = carefully

Here it describes how the recipient reads:

• läser ... noggrant = reads ... carefully

So Swedish uses the adverb form here, just as English uses carefully, not careful.

Yes. Hon means she, and it refers back to mottagaren.

The writer has chosen a female pronoun here, but other options are possible depending on context:

• han = he
• hon = she
• hen = gender-neutral singular
• sometimes people also rewrite the sentence to avoid choosing a gendered pronoun

So hon is not required by the word mottagaren itself. It just shows that the recipient is being referred to as female in this example.

Vad mejlet gäller means something like what the email concerns or what the email is about.

This can feel a bit unusual to English speakers, because English often uses about:

• what the email is about

In Swedish, gälla can mean concern / be about / apply to, and in this pattern you often get:

• vad något gäller

So:

• hon förstår vad mejlet gäller
• = she understands what the email is about

It is idiomatic Swedish. You do not need om here.

Because vad mejlet gäller is an embedded question clause, and embedded clauses in Swedish usually have normal subject-verb order:

• mejlet gäller

So:

• vad mejlet gäller = what the email concerns / is about

If you say vad gäller mejlet, that can mean something different in other contexts, often more like as for the email or it may sound like a different construction entirely.

So in this sentence, vad mejlet gäller is the correct structure.

Yes. Mejl is a standard Swedish spelling for email, and mejlet means the email.

Forms:

• ett mejl = an email
• mejlet = the email
• mejl = emails
• mejlen = the emails

You may also see mail in some contexts, but mejl is the standard Swedish-style spelling.

Yes. A less emphatic version would be:

• Mottagaren förstår vad mejlet gäller först när hon läser ämnesraden noggrant.

That still means basically the same thing:

• The recipient only understands what the email is about when she reads the subject line carefully.

The original sentence is more formal and more focused because it highlights the timing/condition with det är först när ... som ....

Yes. När is the normal choice here because the sentence is talking about the point or condition in time at which understanding happens.

• först när ... = only when ...
• not until ...

So när fits very naturally in this sentence.