diff --git a/prometheus.rules b/prometheus.rules
index 1f75074..b134935 100644
--- a/prometheus.rules
+++ b/prometheus.rules
@@ -62,9 +62,14 @@ groups:
   # the alerts below can be used to keep a closer eye or when starting to use mox,
   # but can be noisy, or you may not be able to prevent them.
 
+  - alert: mox-incoming-delivery-starttls-errors
+    expr: sum by (instance) (increase(mox_smtpserver_delivery_starttls_errors_total[1h])) / sum by (instance) (increase(mox_smtpserver_delivery_starttls_total[1h])) > 0.1
+    annotations:
+      summary: starttls handshake errors for >10% of incoming smtp delivery connections
+
   # change period to match your expected incoming message rate.
   - alert: mox-no-deliveries
-    expr: sum(rate(mox_smtpserver_delivery_total{result="delivered"}[6h])) == 0
+    expr: sum by (instance) (rate(mox_smtpserver_delivery_total{result="delivered"}[6h])) == 0
     annotations:
       summary: no mail delivered for 6 hours
 
diff --git a/smtpserver/server.go b/smtpserver/server.go
index dd87ec7..6e11266 100644
--- a/smtpserver/server.go
+++ b/smtpserver/server.go
@@ -22,6 +22,7 @@ import (
 	"net"
 	"net/textproto"
 	"os"
+	"reflect"
 	"runtime/debug"
 	"slices"
 	"sort"
@@ -59,6 +60,7 @@ import (
 	"github.com/mjl-/mox/smtp"
 	"github.com/mjl-/mox/spf"
 	"github.com/mjl-/mox/store"
+	"github.com/mjl-/mox/tlsrpt"
 	"github.com/mjl-/mox/tlsrptdb"
 )
 
@@ -171,6 +173,21 @@ var (
 			"error",
 		},
 	)
+	metricDeliveryStarttls = promauto.NewCounter(
+		prometheus.CounterOpts{
+			Name: "mox_smtpserver_delivery_starttls_total",
+			Help: "Total number of STARTTLS handshakes for incoming deliveries.",
+		},
+	)
+	metricDeliveryStarttlsErrors = promauto.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "mox_smtpserver_delivery_starttls_errors_total",
+			Help: "Errors with TLS handshake during STARTTLS for incoming deliveries.",
+		},
+		[]string{
+			"reason", // "eof", "sslv2", "unsupportedversions", "nottls", "alert-<num>-<msg>", "other"
+		},
+	)
 )
 
 var jitterRand = mox.NewPseudoRand()
@@ -955,7 +972,25 @@ func (c *conn) cmdStarttls(p *parser) {
 	ctx, cancel := context.WithTimeout(cidctx, time.Minute)
 	defer cancel()
 	c.log.Debug("starting tls server handshake")
+	metricDeliveryStarttls.Inc()
 	if err := tlsConn.HandshakeContext(ctx); err != nil {
+		// Errors from crypto/tls mostly aren't typed. We'll have to look for strings...
+		reason := "other"
+		if errors.Is(err, io.EOF) {
+			reason = "eof"
+		} else if alert, ok := asTLSAlert(err); ok {
+			reason = tlsrpt.FormatAlert(alert)
+		} else {
+			s := err.Error()
+			if strings.Contains(s, "tls: client offered only unsupported versions") {
+				reason = "unsupportedversions"
+			} else if strings.Contains(s, "tls: first record does not look like a TLS handshake") {
+				reason = "nottls"
+			} else if strings.Contains(s, "tls: unsupported SSLv2 handshake received") {
+				reason = "sslv2"
+			}
+		}
+		metricDeliveryStarttlsErrors.WithLabelValues(reason).Inc()
 		panic(fmt.Errorf("starttls handshake: %s (%w)", err, errIO))
 	}
 	cancel()
@@ -971,6 +1006,22 @@ func (c *conn) cmdStarttls(p *parser) {
 	c.tls = true
 }
 
+func asTLSAlert(err error) (alert uint8, ok bool) {
+	// If the remote client aborts the connection, it can send an alert indicating why.
+	// crypto/tls gives us a net.OpError with "Op" set to "remote error", an an Err
+	// with the unexported type "alert", a uint8. So we try to read it.
+
+	var opErr *net.OpError
+	if !errors.As(err, &opErr) || opErr.Op != "remote error" || opErr.Err == nil {
+		return
+	}
+	v := reflect.ValueOf(opErr.Err)
+	if v.Kind() != reflect.Uint8 || v.Type().Name() != "alert" {
+		return
+	}
+	return uint8(v.Uint()), true
+}
+
 // ../rfc/4954:139
 func (c *conn) cmdAuth(p *parser) {
 	c.xneedHello()
diff --git a/tlsrpt/alert.go b/tlsrpt/alert.go
index 3f4ca8b..227bb2a 100644
--- a/tlsrpt/alert.go
+++ b/tlsrpt/alert.go
@@ -10,7 +10,8 @@ import (
 	"strings"
 )
 
-func formatAlert(alert uint8) string {
+// FormatAlert formats a TLS alert in the form "alert-<num>" or "alert-<num>-<shortcode>".
+func FormatAlert(alert uint8) string {
 	s := fmt.Sprintf("alert-%d", alert)
 	err := tls.AlertError(alert) // Since go1.21.0
 	// crypto/tls returns messages like "tls: short message" or "tls: alert(321)".
diff --git a/tlsrpt/alert_go120.go b/tlsrpt/alert_go120.go
index c686a52..a2a396a 100644
--- a/tlsrpt/alert_go120.go
+++ b/tlsrpt/alert_go120.go
@@ -8,6 +8,7 @@ import (
 	"fmt"
 )
 
-func formatAlert(alert uint8) string {
+// FormatAlert formats a TLS alert in the form "alert-<num>".
+func FormatAlert(alert uint8) string {
 	return fmt.Sprintf("alert-%d", alert)
 }
diff --git a/tlsrpt/report.go b/tlsrpt/report.go
index 5e09d33..a25cc37 100644
--- a/tlsrpt/report.go
+++ b/tlsrpt/report.go
@@ -394,7 +394,7 @@ func TLSFailureDetails(err error) (ResultType, string) {
 			// todo: ideally, crypto/tls would let us check if this is an alert. it could be another uint8-typed error.
 			v := reflect.ValueOf(netErr.Err)
 			if v.Kind() == reflect.Uint8 && v.Type().Name() == "alert" {
-				reasonCode = "tls-remote-" + formatAlert(uint8(v.Uint()))
+				reasonCode = "tls-remote-" + FormatAlert(uint8(v.Uint()))
 			}
 		}
 		return ResultValidationFailure, reasonCode
@@ -429,7 +429,7 @@ func TLSFailureDetails(err error) (ResultType, string) {
 		}
 		v := reflect.ValueOf(err)
 		if v.Kind() == reflect.Uint8 && v.Type().Name() == "alert" {
-			reasonCode = "tls-local-" + formatAlert(uint8(v.Uint()))
+			reasonCode = "tls-local-" + FormatAlert(uint8(v.Uint()))
 		}
 	}
 	return ResultValidationFailure, reasonCode