Если письмо снова попадёт в спам, я попрошу отправителя переслать его ещё раз.

Because Russian does not follow the English rule that says after *if, use the present for future meaning*. In Russian, when you are talking about a real future possibility, it is normal to use future forms in both clauses:

• Если письмо снова попадёт в спам, я попрошу...
• literally: If the letter will end up in spam again, I will ask...

This is standard Russian. Compare:

• Если завтра будет дождь, мы останемся дома.
• If it rains tomorrow, we’ll stay home.

So even though English says if it goes, Russian naturally says если попадёт.

Попадёт is the future tense of the perfective verb попасть. It is used because the sentence talks about one specific future event: the email might end up in spam again.

• попадёт = will end up / will get into
• попадает = ends up / gets into in a habitual, general, or present-time sense

So:

• Если письмо снова попадёт в спам... = If the email ends up in spam again...
• Если письмо снова попадает в спам... would sound more like If the email keeps ending up in spam / Whenever it ends up in spam...

Попасть в спам means to end up in spam, to get filtered as spam, or to land in the spam folder.

The verb попасть often means to get into, to end up in, or to land in some place or situation.

Examples:

• попасть в аварию = to get into an accident
• попасть в больницу = to end up in the hospital
• попасть в спам = to end up in spam

Here в спам is a natural modern Russian expression, basically short for the idea of into the spam folder / into the spam category.

Because Если письмо снова попадёт в спам is a subordinate clause introduced by если (if), and in Russian such clauses are normally separated from the main clause by a comma.

So the structure is:

• Если ... , я ...
• If ... , I ...

This comma is required in standard Russian punctuation.

Because попросить takes a direct object in the accusative case: to ask whom?

• попросить кого? = to ask whom?
• отправитель = sender (dictionary form, nominative)
• отправителя = sender in the accusative singular

Since отправитель is a masculine animate noun, its accusative singular looks like the genitive singular:

• nominative: отправитель
• accusative: отправителя

So:

• я попрошу отправителя = I will ask the sender

This is a very common Russian pattern:

• попросить + person (accusative) + infinitive

It means to ask someone to do something.

So:

• я попрошу отправителя переслать его ещё раз
• literally: I will ask the sender to resend it once more

Breakdown:

• я попрошу = I will ask
• отправителя = the sender
• переслать = to resend / to forward
• его = it
• ещё раз = one more time

Russian often prefers this compact infinitive structure. You could also say something with чтобы, but it would be longer:

• Я попрошу отправителя, чтобы он переслал его ещё раз.

That is grammatical, but less direct and less natural here.

Попросить is the perfective partner of просить.

Here the speaker means one completed act of asking in the future, so perfective is the natural choice:

• попрошу = I will ask (once, as a completed act)
• буду просить or просить would suggest a more ongoing, repeated, or process-like meaning

In this sentence, the sequence is:

1. the email ends up in spam
2. then I ask the sender
3. then the sender resends it

All of these are viewed as single completed actions, so perfective verbs fit well.

In this sentence, переслать is the best choice because it means to send again or to forward.

• отправить = to send
• послать = to send too, but often more colloquial and sometimes rougher in tone
• переслать = to resend / forward

The prefix пере- often adds the idea of again or across/forward.

So:

• отправить письмо = to send a letter/email
• переслать письмо = to resend/forward a letter/email

Here the person has already sent the email once, so переслать is exactly right.

Yes, both can translate as again, but they are being used a little differently here.

• снова = again
• ещё раз = one more time / once again

In this sentence:

• снова попадёт в спам = ends up in spam again
• переслать его ещё раз = resend it one more time

Using both sounds natural because they refer to different repeated actions:

1. the email goes to spam again
2. the sender sends it one more time

Also, Russian often varies expressions like this to avoid sounding repetitive.

Его refers to письмо (the email / letter).

Since письмо is the thing being resent, Russian uses the pronoun его = it as the direct object of переслать.

• письмо = email/letter
• переслать его = resend it

Even though письмо is neuter, the object pronoun is still его:

• nominative: оно = it
• accusative: его = it

So this is completely normal:

• Я попрошу отправителя переслать его ещё раз.
• I’ll ask the sender to resend it one more time.

No, Russian word order is fairly flexible, though the version given is very natural and neutral.

The sentence starts with the if-clause, which is common:

• Если письмо снова попадёт в спам, я попрошу...

You could also say:

• Я попрошу отправителя переслать его ещё раз, если письмо снова попадёт в спам.

That means the same thing, but the emphasis is slightly different: the main action comes first.

Within the sentence, the placement of снова and ещё раз is also natural:

• снова is placed near попадёт
• ещё раз is placed near переслать

So the original word order is a good default model to learn.

Yes, in many contexts снова and опять can both mean again:

• Если письмо опять попадёт в спам...
• Если письмо снова попадёт в спам...

Both are grammatical and natural.

A rough tendency:

• снова can sound a little more neutral or formal
• опять can sometimes sound a bit more conversational, and sometimes slightly more emotional, depending on context

In this sentence, снова is a very safe and neutral choice.

Because в here takes the accusative case, and спам is an inanimate masculine noun. For inanimate masculine nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: спам
• accusative: в спам

That is why you see:

• попадёт в спам

and not some different-looking form.

Yes. In modern Russian, письмо can mean either a traditional letter or an email, depending on context.

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

So although письмо literally has the broad sense of letter/message, the context makes it obvious:

• Если письмо снова попадёт в спам... = If the email ends up in spam again...