We are using Postfix as an MTA delivering via Dovecot's LDA (with sieve). We also use Dovecot as a POP/IMAP server. Mail storage is Maildir on NFS, indexes are stored locally. Quotas are FS quotas enforced by the NFS server. The Dovecot version is 1.2.11.
Recently, for one user being over quota, some attempts to deliver mail bounced as expected while some others resulted in zero-sized Maildir files. I can verify zero-sized files in cur, while I cannot tell whether they were already zero-sized in new. However, since nobody except Dovecot is accessing the mail storage, it's either Dovecot's LDA or Dovecot's IMAP producing the zero-sized files.
Logs show successful delivery (deliver: stored mail into mailbox 'INBOX') but ,,Corrupted index cache file /var/db/dovecot/indexes/XXX/.INBOX/dovecot.index.cache: Corrupted virtual size for uid=YYY: 0 != NNNN'' alerts on IMAP access.
It looks like all mails which correctly got bounced are >64KB while those resulting in zero-sized files are <64KB (there is one >32KB). My first guess would be that maybe for large files, write() fails, while for small ones, close() fails or there is some buffering involved for large files.