[VM] turning off auto-save for imap cache folders

[et]

Hi all, I've been experiencing long freezes in emacs, and I
think this happens especially while replying to emails in
vm.  It has been suggested to me that this could be due to
autosave of the large imap folders (16,000 messages, 300Mb).
would anyone know how to turn off auto-save selectively for
the imap folders?  the file name is typically of the form
imap-cache-2cd517ba0be9fc39...  many thanks, E