Hiya Ladies, we found out why this problem was happening, which hopefully our web hosts have now resolved.
The problem was that our web hosts limit the amount of emails being sent per domain name, per hour, and as our forum is getting busier, of course the amount of mail being sent also grew.
This included confirmation emails being sent to new members when registering a new account/membership as well as notification emails being sent automatically by the board letting members know that someone has left a comment or responded to a particular thread - so when the daily email limit had been reached, then the board couldn't access the email server to send the mail and returned a confusing error message.
We've had our daily limits raised a bit but if it becomes an issue again we'll probably have to get our own dedicated server just to keep up with the traffic

Gulp!