What is the purpose of Outbox? Is it just acts like a queue when a message can't be sent immediately for some reason or just holds a message until recipient reads it?
What is the purpose Savebox? Are any messages from inbox will be deleted if not moved there?
Thanks.

If you have messeges in Outbox it means that the user didn't read it yet. I don't know the purpose Savebox

Savebox probably is meant to signify that you've already read it and decided it's worth reading again later. Inbox is usually kept empty for easier notice of new messages. At least, that's how I understand it.

thx for the clarification guys!

well as i see,
while someone sends a message in PM,
sender's outbox is linked to reciever's inbox
both containing the same message.

so i guess if sender deletes the unread message in the outbox, recipiend will not recieve it.

Z