[Dovecot] copy and empty userA folder1, append to /var/mail/userB

[mathog]

Dovecot 2.0.13 on a 64 bit linux.

We have an archiver account on one machine.  Most mailing lists that 
handles go directly to it
and everything is fine.  However, there is one campus list that will 
only send to real people.  So for that
one I subscribed.  When one of these messages comes in, which is 1-2x 
per week, I copy it to an export folder
and then manually run a script which does:

cat ~me/mail/export >>/var/mail/archiver
echo "" > ~me/mail/export

That worked fine on our last system, but dovecot doesn't like it.  It 
leaves the imap indices for the
export mailbox in an inconsistent state, so that roundcube (through 
dovecot) refuses to drop
messages into it.  The error message roundcube emits is:

ServerError:  Mailbox isn't a valid mbox file.

Presumably the second half is from dovecot.

I need a supported way to go about this, so that the next time one 
comes in I can copy to that folder again.
The export mailbox is always completely emptied.  It would suffice if 
there was some way to force dovecot to
create "empty""

.imap/export:
   dovecot.index.cache  dovecot.index.log

files.

Note this isn't a problem for the archiver itself, since it doesn't 
have any imap indexed files.

Suggestions?

Thank you,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech