Junk mail folder
The junk mail folder or spam folder is a location that stores unwanted incoming e-mail messages so that they stay out of a user's inbox. All major e-mail clients and webmail services include a junk folder to help filter and decrease the volume of unnecessary and advertisement-based e-mails.
How does it work?
When you receive a message, your e-mail service looks at the time it was sent, the sender's e-mail address, and the content of the message. If any of these aspects look suspicious, the client filters the e-mail to the spam folder accordingly. E-mail services allow users to decide what should or should not be sent to the junk mail folder by adjusting certain settings.
How to access the junk or spam mail folder
The destination of the junk mail folder varies slightly between each e-mail service and e-mail program, but they're all in a similar area.
Google Gmail
On the left side of the screen, under the Categories section, click the 
icon. You then see an expanded menu with the label Spam.
Yahoo! Mail
As soon as you log in to your Yahoo! e-mail account, you see the selector that allows access to the Spam folder on the left side of the screen.
Outlook.com mail
As soon as you log in to your Outlook.com e-mail account, you see the selector that allows access to the Junk Email folder on the left side of the screen.
If you cannot see the folder list, click 
next to Folders to expand the menu.
iPhone and iPad
If you are using the default mail app on your iPhone or iPad, open the Mail app. Navigate to the Mailboxes view by backing out of the Inbox or any mail folders you are viewing.
In the Mailboxes view, you see the Junk folder below the Drafts and Sent folders.
Gmail smartphone app
When using the Gmail app for an iOS or Android smartphone, select the hamburger menu ☰ button in the top-left corner to view all accessible folders. Select Spam from the list of options.
How long does mail stay in junk mail?
Junk e-mail stays in the junk folder until you are near your storage capacity, exceed a set limit, or the junk mail folder is emptied. Depending on how much e-mail you receive, junk mail can potentially remain in the folder for years. If you receive a lot of e-mail or many e-mails go to the junk folder, they may only stay for a month or two.