I thought one final update on this might help others.
In the end I gave up on Outlook 2010 as the IMAP server.... message was really starting to annoy me. I looked around for alternative email clients and installed Mailbird. It's a bit quirky, and the layout is very different, but I've had no problems sending or receiving email, and copies are being placed in the Sent Items folder consistently.
Problem can be minimised by increasing the Server Timeout.
File - Account Settings - select e-mail acount (double click) - More Settings - Advanced - Server Timeout, set to greatest time (10 minutes).
However, I would like to know how to supress the message from Outlook, as when it pops-up it locks the keyboard until OK is pressed. I don't need to know this information, but there's no option to "not show this message again".