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

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@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