Not able to send PM

[PolishedTopaz]

I have been trying to send a PM to a fellow member, I get a box that says "User does not want PM's to be received or can't receive PM's" I understand the first, but, why would someone not be able to receive a message?
Just wondering

[FrankZ]

If a member's PM box is full they can not receive any further PMs until they delete some.

[Janet H]

Members also can opt to not receive PMs which may be the case.