Errata patches for OpenSMTPD have been released for OpenBSD 6.7. Incorrect use of getpeername(2) storage for outgoing IPv6 connections corrupts stack memory. The nature of the corruption and existing mitigations appear to make this difficult to effectively target.
Binary updates for the amd64, i386, and arm64 platforms are available via the syspatch utility. Source code patches can be found on the errata page: https://www.openbsd.org/errata67.html After patching, restart the smtpd service.