the reply I got from the hosts said the following:
Hi huntlyfc, The outbox messages have been sent , but not viewed yet by the recipients, once the pm is opened and viewed it will no longer appear in the outbox and will then appear in the sentbox. I have many welcome messages unopened in my outbox on my forum as well, It simply means they have not viewed that PM yet.
another tip for all users would be to click on the button at the top of the page which says messages and when the page opens up click on the drop-down select "all posts" then click go which should display the full contents of a users inbox rather than the day or week etc.