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Right click the account and select settings. Under copies and folders, check that the sent is actually set to the sent folder on the account.

Thanks for response - yes this is all in order - I just wondered why there is another folder called sent messages? is this a folder set up by my IMAP server or an old local folder?

unfortunately I can not open the image to view it. All I get is he micro thumbnail.

Ohh it is working today.

you have a send and a sent messages folder. You need to set the sent in the account settings to sent messages not sent on that account.

Once done the sent, if empty, will just disappear, I think. Either way, once the icon changes it can be deleted. A restart of Thunderbird might be required to get it all sorted.