Upgrading from Dovecot Pro 3.1.x to Dovecot Pro 3.1.6

Dovecot Pro

Changed Storage Formats

These changes don't take effect until dovecot_storage_version is changed to 3.1.6.

The dovecot.index.thread index, used by the IMAP THREAD command, has a new v2 format. The Message-ID hashes stored in it are now calculated with a keyed hash using a random per-file key instead of CRC32. This prevents users from crafting Message-IDs whose hashes deliberately collide, which would otherwise slow down threading.

Existing v1 index files are deleted and rebuilt in the v2 format when the mailbox is next opened for threading, so the first THREAD command for a large mailbox can be slower than usual. Dovecot versions older than v3.1.6 don't understand the v2 format and rebuild the index in the v1 format again, so this setting should be increased only once a rollback to an older version is no longer possible.