untrusted comment: verify with openbsd-67-base.pub
RWRmkIA877Io3urz2Ey2QlLpS3IVHlGKyhQMriqt11jLNoKSkNk9fm0XAW/xXuZ/gOdZUpzuWHYSDnTEISckgxK4t4TAV0L6GwM=

OpenBSD 6.7 errata 033, December 24, 2020:

smtpd's filter state machine can prematurely release resources
leading to a crash.

Apply by doing:
    signify -Vep /etc/signify/openbsd-67-base.pub -x 033_smtpd.patch.sig \
        -m - | (cd /usr/src && patch -p0)

And then rebuild and install smtpd:
    cd /usr/src/usr.sbin/smtpd
    make obj
    make
    make install

Index: usr.sbin/smtpd/lka_filter.c
===================================================================
RCS file: /cvs/src/usr.sbin/smtpd/lka_filter.c,v
diff -u -p -u -p -r1.62 lka_filter.c
--- usr.sbin/smtpd/lka_filter.c	24 Apr 2020 11:34:07 -0000	1.62
+++ usr.sbin/smtpd/lka_filter.c	23 Dec 2020 12:58:41 -0000
@@ -597,11 +597,6 @@ filter_session_io(struct io *io, int evt
 		filter_data(fs->id, line);
 
 		goto nextline;
-
-	case IO_DISCONNECTED:
-		io_free(fs->io);
-		fs->io = NULL;
-		break;
 	}
 }