Lots of (hidden?) messages in IMAP inbox

[Jim Burton <jim@sdf-eu.org>]

Hi, I have an IMAP folder and every time I open or update it I get
this message:

Too many messages(2803) in %inbox:jim@mail.sdf-eu.org.  Update all? (y
or n)  

This is annoying because there are only ~200 messages in the inbox and
I have to type 'y' each time to see them. I presume the messages are
in a Trash folder on the server or something like that? How can I find
out where they are and remove them?

The line in .folders for this folder is very simple, like 

%inbox:jim@mail.host.org	"inbox"

Thanks,

Jim