[Dovecot] sent message not always saved in imap sent folder

[Ian Mortimer]

Users who have their sent folder on the imap server report that
sometimes a sent message is saved locally instead of on the server. 
This is the error from evolution:

   Error while performing operation.

   Failed to append to whoever at imap.server:Sent: Server 
   unexpectedly disconnected: Unknown error
   Appending to local `Sent' folder instead..

We're seeing this with more than one client: evolution (linux) and
thunderbird (OS X).  It only happens occasionally but often enough
to be annoying.

It could be because the user's Sent folders are in their home
directories which are NFS mounted.

Are there any dovecot settings which might help?


Thanks
-- 
Ian