Internal queueing error

I had a quick search of the forum but don’t see this one…
No emails sending for a couple of days, incoming is fine. Full error:-
08:45:51   MailClient.Storage.Application.ItemOperationException: Cannot send message (Email body not accepted due to the following reason:
08:45:51     "Internal queueing error
08:45:51 ")

Any suggestions?

ok, looks like this is a server error at my ISP!