Make golangci-lint happy by using errors.As instead of using type assertion on error (which can fail on wrapped errors)

msg.go

@@ -969,12 +969,12 @@ func (m *Msg) HasSendError() bool {
 // SendErrorIsTemp returns true if the Msg experienced an error during the message delivery and the
 // corresponding error was of temporary nature and should be retried later
 func (m *Msg) SendErrorIsTemp() bool {
-	e, ok := m.sendError.(*SendError)
-	if !ok {
-		return false
-	}
+	var e *SendError
+	if errors.As(m.sendError, &e) {
 		return e.isTemp
 	}
+	return false
+}
 
 // SendError returns the senderror field of the Msg
 func (m *Msg) SendError() error {

senderror.go

@@ -5,6 +5,7 @@
 package mail
 
 import (
+	"errors"
 	"fmt"
 	"strconv"
 	"strings"
@@ -94,12 +95,12 @@ func (e *SendError) Error() string {
 
 // Is implements the errors.Is functionality and compares the SendErrReason
 func (e *SendError) Is(et error) bool {
-	t, ok := et.(*SendError)
-	if !ok {
-		return false
-	}
+	var t *SendError
+	if errors.As(et, &t) {
 		return e.Reason == t.Reason && e.isTemp == t.isTemp
 	}
+	return false
+}
 
 // String implements the Stringer interface for the SendErrReason
 func (r SendErrReason) String() string {