Previously catch-all (or “any user”) mail boxes would have the full email address in the Delivered-To header.
The full email address is now stored in the X-Original-To header.
If you were using the Delivered-To header for Sieve or other filtering rules, please update them to use the X-Original-To header instead.
This change is a result of moving our hosted mail internal delivery from Dovecot LDA to the Dovecot LMTP Server in preparation to retire our oldest direct attached storage arrays with a future migration to NFS storage. The LMTP server will not accept <@example.com> mail box names for destinations, so we had to create an internal alias <_default@example.com> for routing. The Delivered-To header now contains this internal alias.