A junk folder is an email client folder dedicated to spam or low priority email messages.
Emails are routed to junk folders according to filtering algorithms from email client softwares and web based email services or according to filtering rules established by users.
Email marketers try to prevent their legitimate opt-in messages to land in junk folders as they are hardly perused by recipients.
In many web-based email services, frontiers are blurring between junk folders and other low priority folders.
Junk folder issue may be considered as a part of email deliverability issue.