Private messaging (pun_pm)

General idea

New table pun_pm_messages

Field	Type	Default	Description
id	int(10)		Auto-increment message ID.
sender_id	int(10)	0	The ID of the sender in users table.
receiver_id	int(10)	NULL	The ID of the receiver in users table.
lastedited_at	int(10)	0	Timestamp of last editing.
read_at	int(10)	0	Timestamp of message reading.
subject	varchar(255)
body	text
status	varchar(9)	draft	One of the 'draft', 'sent', 'delivered', 'read'.
deleted_by_sender	tinyint(1)	0	0 or 1. 1 - deleted by sender.
deleted_by_receiver	tinyint(1)	0	0 or 1. 1 - deleted by receiver.

Additional fields in users table

Field	Type	Default	Description
pun_pm_new_messages	INT(10)	NULL	Stores the cached value of new messages. Values pun_pm_new_messages >= 0 stand for the count of unread messages. pun_pm_new_messages == -1 means that user's Inbox is full. pun_pm_new_messages == NULL means the cache has been cleared and is to be regenerated (for example, when user receives a message).
pun_pm_long_subject	TINYINT(1)	1	0 or 1. 1 - quote the beginning of the message in message lists.

Option: History under the quick reply form.
- Show recent <N> messages from and to the corresponded user.
- Use JS to hide history by default and place the link (Show history) to show it.
Option: Show unread/not delivered messages number in box links (for example Inbox [3] | Outbox [2])
Option: BBCode buttons
Option: Display few usernames (those, who recently has written a message) next to the 'To' field. (JS)
Option: Quote message in the quick reply form.

User option: E-mail notifications (on every message being unread for some period, or once a day)
Option: Threads (messages grouped by topic)
Option: Display avatars
Option: 'This user is writing a letter for you'
Feature: Sort/filter messages*
Feature: Blacklist and disabling PM

When a PM is sent, a popup screen shows as you come into the website and it says something like, “You have new messages” and then have 2 buttons “See Messages” and “Go to forum”