[Exim] exim, cyrus and "plus-addressing"

I am using exim 4.21 with cyrus imap v2.1.16.

I am trying to get "plus addressing" to work.

the relevant section in exim.conf looks like this:

local_cyrus_delivery:
driver = smtp
protocol = lmtp
hosts_override = true
allow_localhost = true

I get no errors when I send mail to myname+Subfolder@???, but
it just ends up in the Inbox, and not in the Subfolder.

Also, I am using "unixhierarchysep=yes"
Any tips appreciated.