nnmaildir and expiry-target

[Desmond Rivet]

Hi all,

I've been trying to set up Gnus so that it would expire my old emails to
different groups of my choosing, calculated off of the source group of
the expired messages.

My reading of the manual suggested that expiry-target or
nnmail-expiry-target was the way to go here, but I simply could not get
this to work.

Then I came across this line from the Gnus manual.  

  6.3.13.6 Group parameters 
  ...
  expire-group
  ...
  Even if this parameter is not set, nnmaildir does not fall back to the
  expiry-target group parameter or the nnmail-expiry-target variable.

Armed with this knowledge, I finally managed to expire my maildir groups
to the target groups of my choosing.

So, my question here is: why is nnmaildir different from the other
backends in this regard?  Why does it not honour the expiry-target or
nnmail-expiry-target settings?  Are there techincal reasons or is it
historical, or what?

Thanks in advance,

-- 
Desmond Rivet

Pain is weakness leaving the body.