Мы не только не нашли письмо в папке «Входящие», но и случайно отправили его в спам.

It means not only ..., but also ....

So:

• не только не нашли письмо в папке Входящие = not only did we not find the email in the Inbox
• но и случайно отправили его в спам = but we also accidentally sent it to spam

This is a very common Russian pairing:

• не только X, но и Y = not only X, but also Y

It is used to connect two parallel ideas, often with a feeling of contrast or escalation.

Because they do two different jobs.

1. The first не belongs to the fixed expression не только ... но и ...

   • не только = not only

2. The second не negates the verb:

   • не нашли = did not find

So не только не нашли literally means:

• not only did we not find

This double не is completely normal in Russian when the first one is part of не только and the second one is actual negation.

Compare:

• Мы не только читали письмо, но и ответили на него. = We not only read the email, but also replied to it.

• Мы не только не читали письмо, но и удалили его. = We not only didn’t read the email, but even deleted it.

They are past tense plural forms, because the subject is мы = we.

Russian past tense agrees with the subject in number and, in the singular, also in gender.

Here:

• мы нашли = we found
• мы отправили = we sent

Past tense endings:

• masculine singular: -л
• feminine singular: -ла
• neuter singular: -ло
• plural: -ли

So:

• я нашёл / нашла
• он нашёл
• она нашла
• оно нашло
• мы нашли

Same pattern with отправить:

• мы отправили

It is in the accusative, but for many neuter inanimate nouns, the accusative looks exactly the same as the nominative.

• nominative: письмо = a letter / email
• accusative: письмо = a letter / email

Since найти takes a direct object, письмо here is accusative.

This is similar to other neuter inanimate nouns:

• вижу окно = I see a window
• нашли место = found a place

The form does not change, but the case still matters grammatically.

Because в can take different cases depending on the meaning.

1. в папке = location → prepositional case

Here it means in the folder, so this is a static location:

• в папке
• в папке Входящие

That is why папка becomes папке.

2. в спам = direction/result → accusative case

With отправить в спам, the idea is movement or transfer into the spam category.

So:

• отправить в спам = to send to spam

This uses the accusative after в.

A useful rule:

• в + prepositional = where? → location
• в + accusative = where to? → destination/result

Compare:

• Письмо лежит в папке. = The email is in the folder.
• Мы положили письмо в папку. = We put the email into the folder.

Входящие is the name of the folder: Inbox.

Literally, it comes from the adjective meaning something like incoming. In email/computer language, Russian often uses adjectives as folder names:

• Входящие = Inbox
• Исходящие = Outbox / Sent outgoing items

It is plural because it originally refers to incoming messages/items.

In this sentence, в папке Входящие means:

• in the Inbox folder

You can think of Входящие as a label or title, so it often stays in this form.

Because его is the object form of the pronoun.

• оно = it as a subject
• его = it as an object (for masculine/neuter nouns)

Since письмо is neuter, the pronoun referring to it is оно in the nominative, but after the verb отправили you need the object form:

• Письмо пришло. Оно важное. = The email arrived. It is important.

• Мы получили письмо и прочитали его. = We received the email and read it.

So in your sentence:

• отправили его в спам = sent it to spam

Случайно means accidentally, by accident, or sometimes unintentionally.

So:

• случайно отправили его в спам = accidentally sent it to spam

It is an adverb, so it describes how the action happened.

Russian word order is flexible, so you might also see:

• ... но и его случайно отправили в спам
• ... но и отправили его случайно в спам

But the version in your sentence is the most natural and clear.

Russian often omits repeated words when they are already clear from context.

The full logical structure is:

• Мы не только не нашли письмо..., но и (мы) случайно отправили его в спам.

The second мы is omitted because it would be redundant.

This is very common in Russian, just as in English:

• We not only didn’t find the email, but also accidentally sent it to spam.

You do not need to repeat we if the subject is obvious.

Both verbs are perfective:

• найти → нашли
• отправить → отправили

Perfective aspect is used because the sentence talks about completed results:

• they did not find the email
• they sent it to spam

The focus is on completed outcomes, not on process.

Compare:

• искали = were looking for / looked for

• нашли = found

• отправляли = were sending / used to send
• отправили = sent

In this sentence, perfective is the natural choice because it describes what happened in a specific situation.

It can mean both, depending on context.

• письмо = letter
• письмо = email

Because the sentence mentions Входящие and спам, the meaning is clearly email here.

Russian often uses the same word for both traditional and electronic mail, just like English sometimes uses message in different contexts.

Yes, Russian word order is flexible, but not every version sounds equally natural.

The original sentence is very natural:

• Мы не только не нашли письмо в папке Входящие, но и случайно отправили его в спам.

This order works well because:

1. Мы introduces the subject clearly.
2. не только ... но и ... creates a neat parallel structure.
3. случайно appears close to the verb it modifies.

A different order might still be grammatical, but could sound more marked or emphasize something different.

For example:

• Мы письмо не только не нашли в папке Входящие, но и случайно отправили его в спам.

This puts extra emphasis on письмо.

So yes, the order can change, but the original is probably the best model for a learner to copy.