If you have spam filtering turned on in ISPConfig, then each message is marked as spam or not based on a large set of rules. Each message is assigned a score and declared to be spam if it reaches a minimum score. You have the option of modifying the subject line of the email to indicate if it's spam. (All email is run through a spam filter and very high scoring spam is rejected.)
If you log into ISPConfig using your email address and password, you can change your spam filtering settings: whether the subject is modified and how; which emails are blacklisted and which are whitelisted, and the threshold for declaring an email to be spam.
Part of the spam filter is a "Bayesian" filter that can be trained to recognize spam. You can improve spam recognition by moving spam messages into your "Junk" folder on the mail server. If you are reading your email via an IMAP connection (from Microsoft Outloook, Mozilla Thunderbird, or many smart phone email programs, etc.) you can simply move the message into the Junk folder. You can also move messages in web mail. For example, you can log into our RoundCube web mail system and scan through your Inbox. Select any obvious spam messages and then select "Mark as Junk" from the menu.
You can also scan through the Junk folder and move any messages that were incorrectly flagged as spam back to your inbox.