Emails are just files.
Every email account has an Inbox and Imap Spam Folder . When someone sends you an email it is first scanned by the anti spam systems Magicspam and Spam Assassin and emails that are identified as spam are flagged with ***SPAM *** in the subject.
Spam filters by default put email marked as SPAM into the mailboxes's Spam folder which is technically an IMAP sub folder OR you can set each of the anti spam systems spam systems to leave your email flagged as SPAM in your inbox.
Every customer chooses what system they use to read / send email typically : mobile phones, computer email clients like windows mail, apple mail, outlook, thunderbird or webmail at webmail.yourdomain or external webmail reader like google.
Some systems only permit using IMAP to read emails but some do permit POP3 which is an older email protocol.
You decide how to configure your device ie: mobile phone or comuter or tablet to connect your server and depending upon how your email system is configured will determine the your options:
If your email system is using IMAP it connects to the mail account and your system can view email in the main inbox or its spam sub folder.
If your email system is set to use POP3, it does NOT have the ability to read IMAP folders, it connects to the email accounts INBOX only and downloads the email to your local computer and deletes it from the server after it's completed download therefore if you want to view spam you must either set your spam filters to leave email marked as SPAM in the inbox and then setup a routing filter within your software to move emails with SPAM in the subject to your own system spam folder or use webmail or another IMAP compliant system to connect and read the spam folder since your main email client won't be able to view them using POP3.
Our email servers are self managed and you choose how you want to read your email what protocols to use and you can adjust the spam routing in the spam filter or magic spam connection filter to leave mail in your inbox or move to spam folder.