Я заглянула в архив, но письмо почему-то оказалось в спаме.

Because the speaker is female.

In the past tense, Russian verbs agree with the subject in gender and number:

• заглянул = masculine
• заглянула = feminine
• заглянуло = neuter
• заглянули = plural

So Я заглянула tells you that the person saying я is a woman.

It could be omitted if the context is clear.

Russian often drops subject pronouns because the verb form already gives a lot of information. Since заглянула already shows first person singular and feminine, Заглянула в архив... can be enough in context.

Including я makes it a bit more explicit or contrastive, but it is still very natural.

Literally, заглянуть can mean to glance in, to peek into, or to look in. In this sentence, it has a very natural extended meaning: to check briefly or to take a quick look.

So Я заглянула в архив means something like:

• I checked the archive
• I had a quick look in the archive

It sounds lighter and more casual than a verb meaning a long, careful search.

Because after заглянуть, Russian normally uses в + accusative: заглянуть во что? — в архив.

The idea is that your attention is directed into something. Even though there may not be physical movement in English, Russian treats it like looking into a space/container.

Compare:

• заглянуть в архив = to look into / check the archive
• быть в архиве = to be in the archive
• найти в архиве = to find in the archive

So:

• в архив = direction / looking into it
• в архиве = location inside it

Here it almost certainly means an email archive or archive folder, not a physical archive of documents.

Russian often uses short, context-based nouns like this in tech or app contexts. So архив can mean:

• an archive in the general sense
• the Archive folder in email or messaging

In this sentence, because of письмо and спам, the digital meaning is the natural one.

Почему-то means for some reason, somehow, or for reasons unknown.

It suggests that the speaker does not really know why something happened, or finds it a bit odd/unexpected.

So:

• почему = why?
• почему-то = for some reason

In this sentence, it adds the idea that the email ended up in spam unexpectedly:

• but the message somehow / for some reason turned out to be in spam

Because письмо is neuter singular, and the past tense of оказаться agrees with it.

• письмо is neuter
• therefore: письмо оказалось

Compare:

• документ оказался = the document turned out...
• письмо оказалось = the letter/email turned out...
• папка оказалась = the folder turned out...

So the verb form is matching the grammatical gender of письмо, not the gender of the speaker.

Not exactly. Оказаться often means to turn out to be, to prove to be, or to end up being found in some place or state.

So письмо оказалось в спаме is not just a plain statement of location. It has an element of discovery or surprise:

• the email turned out to be in spam
• the email was found to be in spam
• the email ended up in spam

That nuance is important: the speaker checked one place, but then discovered the real situation.

It is prepositional case after в because this clause describes location:

• в спаме = in spam

The noun спам is treated as a normal masculine noun in Russian:

• nominative: спам
• prepositional: в спаме

So this follows the standard pattern:

• в архиве
• в письме
• в спаме

Even though English often says in spam or in the spam folder, Russian naturally says в спаме.

Because Russian often leaves out words that are obvious from context.

In email language, архив and спам can stand by themselves and mean:

• the Archive folder
• the Spam folder

So письмо оказалось в спаме is a very natural, compact way to say the email was in the spam folder.

If you wanted to be more explicit, you could say something like в папке Спам, but that is not necessary here.

By dictionary meaning, письмо can mean either letter or email.

In this sentence, the context clearly points to email, because of:

• архив
• спам

So even though the word itself is broader, a Russian speaker would naturally understand it here as an email message.

Но means but, and it marks a contrast between expectation and reality.

The speaker checked the archive, but the message was not there after all; it turned out to be in spam. So но connects:

• what the speaker did or expected
• what was actually true

It gives the sentence a very natural I looked in one place, but the real answer was different feeling.

The word order is natural and conversational.

Я заглянула в архив, но письмо почему-то оказалось в спаме.

A rough information flow is:

1. what the speaker did
2. contrast
3. what was discovered

Placing письмо early in the second clause makes it the topic of that clause: as for the email, it turned out to be in spam.

Russian word order is flexible, but this version sounds smooth and neutral.