Each user can specify via their User CP whether or not they'd like to receive PMs. Users can also add people to their Ignore list to avoid receiving messages from them. As tidy rabbit suggested, the PM box can also be full, which prevents reception of additional messages.
I actually don't know what the specific text of the message someone receives for the above different circumstances, but if you let us know the message you received, we can let you know.
has chosen not to receive private messages or may not be allowed to receive private messages. Therefore you may not send your message to him/her.
Usually in the past it will tell me the mail box is full.
Either they have their settings set to not use PMs or administration has not activated or has deactivated their ability to use PMs. Since this board's administration does not restrict PMs on anyone for any reason, and every member has access to using PMs if they only turn them on, their PM account is not restricted because of Admin. An individual, however, can turn off their PM usage.
That, as Ambrey seems to know, might also be the message you receive if you send a PM to someone who has you on ignore. As I have never experienced trying to post to anyone who has me on ignore, I cannot say what the "on ignore" notice is that you get. Either the person turned off their PMs altogether, or they have you on ignore.
Airborne? Oh. Yes, he can take a joke. Once. After that, the joke's on you.
H2, they may have accidentally turned off their PMs...I did that once and didn't realize it. If they have a full PM box the message back to the sender usually says something like that person's message box is full.
You jump in the saddle,
Hold onto the bridle!
Jump in the line!
For those who may not be aware, you can change your PM (and other account) settings by clicking on your User CP from the left side of the light blue bar at the top of the page.
Click on Edit Options and scroll down to Private Messaging options to specify whether or not you'd like to receive them (or just receive them from administrators and members of your buddy list, if you have one) and what kind of notification you receive when a PM arrives.