Errata patches for OpenSMTPD have been released for OpenBSD 6.4. The mail.mda and mail.lmtp delivery agents were not reporting temporary failures correctly, causing smtpd to bounce messages in some cases where it should have retried them.
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/errata64.html After patching, restart the smtpd service.
