After several testing, it is confirmed that why our campaign emails went to some recipients’ (mainly corporate recipients) junk folders is because of the open view tracking. I have to comment out a line in EmailMan.php to get the emails landed in their inbox.
https://suitecrm.com/suitecrm/forum/suitecrm-7-0-discussion/2554-email-campaigns-x-brightmail-1-00-x-cp3a-suspected-spam#67000
I like to suggest adding an option to disable those auto tracking e.g. the open view tracking and click-through tracking.