Hi,
Postfix has this issue as well. So does qmail. So does exim. It has nothing to do with the software being used. It is a problem in the NFS protocol.
Just to be clear. Of course these programs wont have this issue when used with dovecot-imap, because obviously they wont be updating any dovecot index files. You can tell dovecot-lda to not update index files, and you'd have exactly the same behavior. Combine that with not allowing multiple logins, and indeed, you wont ever need the director.
What I meant was, if qmail, or exim, or postfix, would use some kind of indexing like dovecot, and would allow that over NFS, they'd have the same underlying issue to deal with. It's actually quite amazing and cool that Timo has addressed this problem for those that are using dovecot-lda with indexing over NFS, and/or dovecot-imap with multiple logins by the same user over NFS.
Cor