Questions & Answers about 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