Иногда важные письма попадают в спам.

Попадают here means end up in, get into, or land in spam.

The verb is попадать / попасть, which often has the basic idea of getting into a place/situation, sometimes accidentally or unexpectedly.

So in this sentence:

• письма попадают в спам = emails end up in spam

It does not suggest that the emails physically fall somewhere. It is the normal Russian way to say that something gets routed into the spam folder.

Because the subject is plural:

• письма = letters / emails (plural)

So the verb must also be plural:

• письма попадают = emails end up

Compare:

• письмо попадает в спам = An email ends up in spam
• письма попадают в спам = Emails end up in spam

Because both words are in the nominative plural, which is the form used for the subject of the sentence.

• письмо = letter/email
• письма = letters/emails

And the adjective must agree with the noun:

• важный = important (masculine singular)
• важные = important (plural)

So:

• важные письма = important emails

Both words are plural because the sentence is talking about multiple emails.

This is because Russian often uses:

• в + accusative for motion into
• в + prepositional for location inside

Here the idea is that the emails go into spam, so Russian uses в спам.

Compare:

• Письма попадают в спам. = Emails go into/end up in spam.
• Письма уже в спаме. = The emails are already in spam.

So:

• в спам = into spam
• в спаме = in spam

Because спам is a masculine inanimate noun, and in the accusative singular, masculine inanimate nouns usually look the same as the nominative.

So:

• спам (nominative)
• в спам (accusative, same form)

That is normal.

A learner might expect a visible ending change after a preposition, but in this case there is none.

Literally, письма can mean either letters or emails, depending on context.

But because the sentence mentions спам, the natural meaning here is emails.

So even though the Russian word is the same, the context makes it clear:

• важные письма попадают в спам = important emails end up in spam

Иногда means sometimes. Putting it first is very natural because it sets the time/frequency frame for the whole sentence.

• Иногда важные письма попадают в спам.

This is similar to English:

• Sometimes important emails end up in spam.

Russian word order is flexible, so other orders are possible, for example:

• Важные письма иногда попадают в спам.

That also works. Starting with Иногда just sounds very natural and neutral.

In Russian, the present tense is often used for habitual, repeated, or general situations.

So:

• попадают literally looks like they get/end up
• but in context it means sometimes they end up / they sometimes end up

Because of Иногда, the sentence describes something that happens from time to time, not necessarily right now.

This is completely normal in Russian.

Попадать is the imperfective verb, and it is used here because the sentence describes a repeated / habitual event:

• Иногда важные письма попадают в спам.
• Sometimes important emails end up in spam.

This means it happens from time to time.

The perfective попасть is used more for a single completed event:

• Важное письмо попало в спам.
• An important email ended up in spam.

So the contrast is:

• попадают = happen repeatedly / sometimes
• попало = happened once

Yes, in practice в спам often means into the spam folder.

Russian commonly uses спам by itself in this context, just as English speakers often say:

• It went to spam instead of
• It went to the spam folder

A fuller version would be something like:

• в папку “Спам” = into the Spam folder

But in everyday language, в спам is the normal concise way to say it.

No, not necessarily. The sentence is focused on the result: the emails end up there.

It does not name an agent such as:

• the email service
• the spam filter
• a person

So the feel is more like:

• important emails sometimes get sent to spam
• important emails sometimes end up in spam

If you wanted to emphasize the system doing it, Russian might use a different construction, for example with фильтр or a passive-style wording.

Yes. Several English translations work well, depending on style:

• Sometimes important emails end up in spam.
• Sometimes important emails go to spam.
• Sometimes important emails get sent to spam.

The most literal-feeling match for попадают is probably:

• end up in spam or
• go to spam

If you want the most natural everyday English, end up in spam is often the best choice.

It is quite neutral and natural.

• Иногда = standard
• важные письма = standard
• попадают в спам = very common modern usage

It works well in everyday speech, help articles, tech support, and general conversation.

So this is a very useful pattern to remember:

• что-то попадает в спам = something ends up in spam