nnimap-split-rule to non-imap backend?

[Andrew Telford <telford.andrew.aj@bhp.com.au>]

The help for nnimap-split-rule says:

  The first element can also be a list. In that case, the first
  element is the server the second element is the group on that server
  in which the matching article will be stored.

Does "server" here refer to an imap server hostname or something like
"nnml"?  It would be great to be able to have

(setq nnimap-split-rule '(('(nnml "gnus-imap")   "From:.*gnus-imap")
			  ("INBOX.junk"        "Subject:.*buy")))

to have high-volume mailing lists automatically sucked off the imap
server onto the local disk.  When I tried this it complained that
something, perhaps '(nnml "gnus-imap"),  was not a string.

I tried the alternative strategy of setting an imap "mail-source" but
this did not delete files off the imap server after copying them to a
local backend, in this case, nnml.  (Even with nnmail-delete-incoming
= t).

We only have a limited quota on our imap server (Exchange) so I need to
have mail split to local disk with the mail deleted on the server.

Any suggestions,

Andrew

using imap from "Pterodactyl Gnus v0.98"