What are they called? This sounds like there is a problem. Their names might tell us a bit about what's going on.

Thanks for reply.

Folders start with 'mail' then 'my domain name' then 'co' then 'uk' then 'archives' then '2015' then 'junk' then 'sent'

Another set start with 'trash' then 'my domain name' then 'co' then 'uk' then 'archives' then 'drafts' then 'junk' then a couple of my contacts folders then 'sent'

Some folders contain messages. The new sent folders contain messages - but not the same as each other - and not the same as the general sent folder. VERY CONFUSING

perhaps you could post a screen capture of that, it is rather difficult to visualize.