Admin Control Panel > vBulletin Options > vBulletin Options > Private Messaging Options
Private Messaging Enabled (yes/no)
Turns the entire private messaging system on and off.
Instant Messaging Support - Check for New Private Messages (yes/no)
Selecting 'Yes' for this option will cause the system to check the private message database every time a user loads a page, and will display a visible prompt if a new message has just been saved.
Maximum Characters Per Private Message (default: 5000)
Maximum characters to allow in a private message.
Set this to 0 for no limit.
Floodcheck - Minimum Time Between Messages (default: 60)
Private Message Flood Checking. Select the minimum time that must pass before a user can send another private message. This is to prevent a single user 'spamming' by sending lots of messages very quickly.
Set this to 0 to disable the option.
Default Messages Per-Page (default: 50)
This setting allows you to define the default number of messages displayed per-page on the private messages listings pages.
Maximum Messages Per-Page (default: 100)
This setting allows you to limit the number of messages users may display per page on the private messages folder view pages.
Allow Message Icons for Private Messages (yes/no)
Allow the use of the standard message icons for private messages.
Allow vB Code in Private Messages (yes/no)
Allow users to include vB Code in their Private Messages? (Such as [b], [i] etc.)
Allow Smilies in Private Messages (yes/no)
Allows users to include smilies in their Private Messages.
Allow [IMG] Code in Private Messages (yes/no)
Allows users to include [IMG] codes in their Private Messages.
Allow HTML in Private Messages (yes/no)
Allow users to include raw HTML code in their Private Messages?
(Strongly not recommended.)