1. How system finds spam email?
--- Spam blocker reads header of email and compare with preset spam level limit. And if this value equals or cross then message will be found as spam.
2. What to do if system detects spam?
-- Here you will have optios.
1. Mark Email As Spam and and pass to mail box. (This will edit email subject with "Spam")
2. Mark Email as spam and quarantine. Here admin 'll review message header and then release to deliver if it is not a spam
3. Delete Spam : This will delete spam emails and email wont exists longer.
so it is hard to decide is it spam or not then deliver to mailbox. Not possible for wide range of emails.
Following is good practice for Spm Blocking.
1. Set fix spam header value. (This needs some exercise for trial and error)
2. Check with Email Server KB and Configuration guide.
3. Set action : Mark spam and deliver to mailbox.
I have used this and had good result. This may help you out.
There are many factors that can influence deliverability and nolimitsoldier mentioned some of them. Others can include bounce handling, spam trap hits, rate limit compliance, spam reports, and list management.
Increase email deliverability and know who clicked and opened your emails.