what happens to dropped emails?!

very helpful indeed

The sending mail server will try to deliver it and it will fail, it will not get a bounce back if it is truly dropped and you will have no copy of it.  You need to set it either to accept and quarantine (you will get a lot of junk) or just trust it.  It does seem a little mad but unless you know you are losing genuine emails I wouldn't worry too much.

i just need to catch the false positive .. how to set all the dropped messages to be quarantined ? i use pop3

As far as I can see when retrieving emails over pop emails won't be dropped. You can only accept mails or add a prefix to the subject.

so i can be sure that all incoming emails are not lost right ?!

I'm pretty sure that most of the settings belong to smtp communication. It seems that pop doesn't drop mails. And from my point of view this wouldn't make sense because the mails then would stay in the mailbox until it is full. So I would say pop does not drop any mails.