diff --git a/cmd/prometheus/main.go b/cmd/prometheus/main.go
index cf25622c9..5b3048cb1 100644
--- a/cmd/prometheus/main.go
+++ b/cmd/prometheus/main.go
@@ -174,7 +174,7 @@ func main() {
 	a.Flag("rules.alert.for-grace-period", "Minimum duration between alert and restored 'for' state. This is maintained only for alerts with configured 'for' time greater than grace period.").
 		Default("10m").SetValue(&cfg.forGracePeriod)
 
-	a.Flag("rules.alert.resend-delay", "Minimum amount of time to wait before resending an alert to Alertmanager. Must be lower than resolve_timeout in Alertmanager").
+	a.Flag("rules.alert.resend-delay", "Minimum amount of time to wait before resending an alert to Alertmanager.").
 		Default("1m").SetValue(&cfg.resendDelay)
 
 	a.Flag("alertmanager.notification-queue-capacity", "The capacity of the queue for pending Alertmanager notifications.").
@@ -700,6 +700,8 @@ func sendAlerts(n *notifier.Manager, externalURL string) rules.NotifyFunc {
 			}
 			if !alert.ResolvedAt.IsZero() {
 				a.EndsAt = alert.ResolvedAt
+			} else {
+				a.EndsAt = alert.ValidUntil
 			}
 			res = append(res, a)
 		}
diff --git a/rules/alerting.go b/rules/alerting.go
index a542f89e7..a337936e0 100644
--- a/rules/alerting.go
+++ b/rules/alerting.go
@@ -89,6 +89,7 @@ type Alert struct {
 	FiredAt    time.Time
 	ResolvedAt time.Time
 	LastSentAt time.Time
+	ValidUntil time.Time
 }
 
 func (a *Alert) needsSending(ts time.Time, resendDelay time.Duration) bool {
@@ -440,11 +441,17 @@ func (r *AlertingRule) ForEachActiveAlert(f func(*Alert)) {
 	}
 }
 
-func (r *AlertingRule) sendAlerts(ctx context.Context, ts time.Time, resendDelay time.Duration, notifyFunc NotifyFunc) {
+func (r *AlertingRule) sendAlerts(ctx context.Context, ts time.Time, resendDelay time.Duration, interval time.Duration, notifyFunc NotifyFunc) {
 	alerts := make([]*Alert, 0)
 	r.ForEachActiveAlert(func(alert *Alert) {
 		if alert.needsSending(ts, resendDelay) {
 			alert.LastSentAt = ts
+			// Allow for a couple Eval or Alertmanager send failures
+			delta := resendDelay
+			if interval > resendDelay {
+				delta = interval
+			}
+			alert.ValidUntil = ts.Add(3 * delta)
 			anew := *alert
 			alerts = append(alerts, &anew)
 		}
diff --git a/rules/manager.go b/rules/manager.go
index 2a9ac3397..baf18c959 100644
--- a/rules/manager.go
+++ b/rules/manager.go
@@ -393,7 +393,7 @@ func (g *Group) Eval(ctx context.Context, ts time.Time) {
 			}
 
 			if ar, ok := rule.(*AlertingRule); ok {
-				ar.sendAlerts(ctx, ts, g.opts.ResendDelay, g.opts.NotifyFunc)
+				ar.sendAlerts(ctx, ts, g.opts.ResendDelay, g.interval, g.opts.NotifyFunc)
 			}
 			var (
 				numOutOfOrder = 0
diff --git a/rules/manager_test.go b/rules/manager_test.go
index 66c33fe64..429fcbb05 100644
--- a/rules/manager_test.go
+++ b/rules/manager_test.go
@@ -689,6 +689,7 @@ func TestNotify(t *testing.T) {
 	// Alert sent right away
 	group.Eval(ctx, time.Unix(1, 0))
 	testutil.Equals(t, 1, len(lastNotified))
+	testutil.Assert(t, !lastNotified[0].ValidUntil.IsZero(), "ValidUntil should not be zero")
 
 	// Alert is not sent 1s later
 	group.Eval(ctx, time.Unix(2, 0))