What Is the Spam Folder?
TL;DR
The spam folder, sometimes called the junk folder, is where a receiving provider puts mail it accepted but judged unwanted. Delivery succeeded at the SMTP level, so nothing bounces and the sending log records a success, which is why spam placement is invisible from the sending side.
How do you know mail is going to spam?
Not from your sending reports, which will look healthy. The symptom is behavioural: replies stop while delivery stays clean, and the drop is often concentrated at one provider rather than spread evenly, because each provider decides separately.
Confirming it takes a view from the receiving side. Seed mailboxes at the providers on your list show the folder directly, Google Postmaster Tools shows Gmail reputation and spam rate for your domain, and a friendly recipient who checks their junk folder is a perfectly valid one-off test.
How do you get out of the spam folder?
In the order that fixes causes rather than symptoms. Correct authentication so identity is not in question, cut volume to what the recent history supports, and stop sending to the segment that generated the complaints and bounces, because that segment is what taught the filter its current opinion.
Then wait, and keep sending a smaller amount of mail that people answer. Provider reputation is computed over a rolling window of recent behaviour, so recovery is the arrival of a better window rather than a setting anyone can change, and messages asking recipients to move you out of spam mostly annoy people who already ignored you.
Frequently asked questions
Yes, and it is one of the stronger positive signals available, because a user of that provider is overruling the filter of that provider. It helps most for that recipient and contributes to domain reputation more broadly, though a handful of rescues will not offset a bad sending pattern.