[Exim] msglog directory keeps files around

I'm using Exim 4.30 together with MailScanner 4.25, with a split
spool-directory setup as described in the second part of the docs
(http://www.sng.ecs.soton.ac.uk/mailscanner/install/exim.shtml)

Having noticed that the message logs obviously remain in exim_in/msglog
after the message has been moved by MailScanner into exim_out/input
I created a symbolic link between the two msglog/ directories, thinking
I'd be doing "the right thing". Unfortunately, I'm not; a few weeks later,
I now find many thousand files in msglog/ pertaining to messages which
have long been processed and delivered (i.e. no corresponding files
in input/ for that message-id).

My config file contains these relevant entries:

    SPOOL_DIRECTORY         = /var/spool/exim_out
    spool_directory         = SPOOL_DIRECTORY
    timeout_frozen_after    = 14h
    auto_thaw               = 1h
    ignore_bounce_errors_after = 2h

Is there something missing there, or do I need to delete those files
manually ?

Thanks & regards,
    -JP