You can voluntarily find SpamAssassin in use in both email clients and servers and in different operating systems that require filtering incoming as well as outgoing email, and implementing a very broad range of policy actions. To help you comprehend it check these process which will help you to configure Spam Assassin.:

required_hits - The number of e-mails received before marked as spam.
rewrite_subject - The tells Spam Assassin if it should rewrite your subject line or not as you define. (1 is yes and 0 is no)
subject_tag - What the spam is marked with in the subject line. (usually ***SPAM*** or something else that could be picked up by your own mail filters on your mail client of choice)

blacklist_from - An e-mail address you definitely want to be marked as spam. (you could also use *@sexample.com to get every email from that address filtered)

whitelist_from - An e-mail address you definitely do NOT want to be marked as spam. (you could also use *@example.com to get every email from that address NOT filtered)

Once you’re done, just hit save and whatsoever you defined on the earlier page will now be found effective. If you picked to rewrite the subject, then you could now go into your mail client and write a new “rule” that would filter all mail with “***SPAM***” to go into a SPAM folder so that you could check it time to time making sure you wont get a hold to get any false positives.