[exim] .forward to mbx folders?

Hello,

I have set up .forward files to farm mail into different folders with
this sort of rule:

#Exim filter
if
$h_to: contains "exim-users@???" or $h_cc: contains
"exim-users@???"
then
save $home/.imap/Lists/Exim
endif

The problem is these files are all in MBX format and exim doesn't
automatically detect this. Adding mbx_format = true to the .forward
file doesn't work. So I thought I'd change the address_file transport
and add the following line (I could make this more intelligent and check
if a file exists or something later):

mbx_format = ${if eq {$local_part}{mike} {yes}{no} }

However this fails with:

Exim configuration error in line 622 of /etc/exim/configure:
"" is not a valid value for the "mbx_format" option

So how can I make exim save use mbx format for some users or files and
not for others? What am I doing wrong? Where do babies come from? So
many questions...

Running Exim 4.41 on OpenBSD 3.6 btw

Thanks
Mike