Dovecot quota enforcement on NFS-mounted mail directories

The documented quota_rule capability sets mailbox size limits that Dovecot evaluates using file-system metadata. On a local ext4 volume the check is reliable and deliveries are rejected once the limit is reached.

When the same mail directory is hosted on an NFS share the metadata can become stale or unsupported, causing the quota plugin to silently skip the size check. The configuration appears valid and produces no errors, yet mailboxes grow beyond the declared limit in production.

This behavior is a known limitation of the quota plugin across Dovecot 2.4.x releases; the core algorithm has not changed. NFS server options such as noatime or sync may influence the severity, but no configuration flag forces a re-check against the remote filesystem.

What is the recommended approach to obtain reliable quota enforcement when mail storage must reside on NFS? Are there alternative quota backends (e.g., dict quota with a database) that avoid the metadata dependency? Does Dovecot expose any diagnostic signal when a quota check is skipped due to filesystem limitations?