diff --git a/spindle/db/mill_state.go b/spindle/db/mill_state.go
index 95310fcf..1524e2aa 100644
--- a/spindle/db/mill_state.go
+++ b/spindle/db/mill_state.go
@@ -123,7 +123,12 @@ func (d *DB) AppendOutboxRow(payload []byte, control bool) (uint64, error) {
 
 	var epoch string
 	var nextSeqno uint64
-	err = tx.QueryRow(`select epoch, next_seqno from mill_outbox_state limit 1`).Scan(&epoch, &nextSeqno)
+	err = tx.QueryRow(`
+		update mill_outbox_state
+		set next_seqno = next_seqno + 1
+		where epoch = (select epoch from mill_outbox_state limit 1)
+		returning epoch, next_seqno - 1
+	`).Scan(&epoch, &nextSeqno)
 	if err == sql.ErrNoRows {
 		return 0, fmt.Errorf("no outbox epoch set")
 	} else if err != nil {
@@ -143,13 +148,6 @@ func (d *DB) AppendOutboxRow(payload []byte, control bool) (uint64, error) {
 		return 0, err
 	}
 
-	if _, err := tx.Exec(
-		`update mill_outbox_state set next_seqno = ? where epoch = ?`,
-		nextSeqno+1, epoch,
-	); err != nil {
-		return 0, err
-	}
-
 	if err := tx.Commit(); err != nil {
 		return 0, err
 	}
diff --git a/spindle/db/mill_state_test.go b/spindle/db/mill_state_test.go
index c4990b9d..9902c78c 100644
--- a/spindle/db/mill_state_test.go
+++ b/spindle/db/mill_state_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"path/filepath"
+	"sync"
 	"testing"
 
 	"tangled.org/core/notifier"
@@ -268,6 +269,57 @@ func TestOutboxPrefixAck(t *testing.T) {
 	}
 }
 
+func TestOutboxConcurrentAppend(t *testing.T) {
+	d := newTestDB(t)
+	if err := d.SetOutboxEpoch("inc-1"); err != nil {
+		t.Fatalf("SetOutboxEpoch: %v", err)
+	}
+
+	const count = 32
+	type result struct {
+		seqno uint64
+		err   error
+	}
+	start := make(chan struct{})
+	results := make(chan result, count)
+	var wg sync.WaitGroup
+	for i := range count {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			<-start
+			seqno, err := d.AppendOutboxRow([]byte(fmt.Sprintf("msg%d", i)), true)
+			results <- result{seqno: seqno, err: err}
+		}()
+	}
+	close(start)
+	wg.Wait()
+	close(results)
+
+	seen := make(map[uint64]struct{}, count)
+	for result := range results {
+		if result.err != nil {
+			t.Errorf("AppendOutboxRow: %v", result.err)
+			continue
+		}
+		if _, ok := seen[result.seqno]; ok {
+			t.Errorf("duplicate outbox seqno %d", result.seqno)
+		}
+		seen[result.seqno] = struct{}{}
+	}
+	if len(seen) != count {
+		t.Fatalf("successful appends = %d, want %d", len(seen), count)
+	}
+
+	rows, err := d.ListOutboxRows()
+	if err != nil {
+		t.Fatalf("ListOutboxRows: %v", err)
+	}
+	if len(rows) != count {
+		t.Fatalf("persisted outbox rows = %d, want %d", len(rows), count)
+	}
+}
+
 func TestCompositeCursors(t *testing.T) {
 	d := newTestDB(t)