
This is undoubtedly due to the recipient's INBOX being full up!
Let's just explain the PM facility.
You have 4 'boxes'
1. INBOX
2. SENT BOX
3. OUTBOX
4. SAVE BOX
Now each of those red boxes can and will hold 60 messages. Once theyre full, you aint gonna be sending messages.
What you need to do, is now and again EMPTY them!
SENT BOX just scroll to the bottom and click 'DELETE ALL' and that's that one sorted, unless you wish to keep anything for reference, in which case manually select the messages and click 'SAVE MARKED'.
After that then Click 'DELETE ALL'
IN BOX Simply do the same again, save what you want, delete the rest!
SAVE BOX With the Save Box you are stuck with 60 messages. I can alter the amount you can save and I have done twice now, but I point blank refuse to enlarge the storage space any more than it's current levels. It's down to you lot to do a bit of housekeeping.

There ya go. A quick Geek on RSPMS

To summarise ; TIDY YOUR PM BOXES UP!!
