I am using Thunderbird and this has got me totally lost.
Over 900 e-mails from back in 2005 and 2006, have come back into my
inbox as messages unread.
These were e-mails that were read & deleted and then emptied out of
the trash bin.

I now cannot delete them as a pop-up window comes up to stating," the
folder trash is full and can't hold any more messages. To make more
room delete any old or unwanted e-mails and Compact the folder."

Please help, Geoff.