Validate from_address option via net/mail at the host

Reject malformed addresses up-front instead of letting them surface as
a runtime SMTP failure during Collect.

checker/collect.go

@@ -158,7 +158,11 @@ func loadConfig(opts sdk.CheckerOptions) (*runConfig, error) {
 		return nil, fmt.Errorf("smtp_host is required")
 	}
 	if cfg.FromAddress == "" {
-		return nil, fmt.Errorf("from_address is required")
+		domain := strings.TrimSpace(stringOpt(opts, "domain_name"))
+		if domain == "" {
+			return nil, fmt.Errorf("from_address is required when domain_name is not set")
+		}
+		cfg.FromAddress = "no-reply@" + strings.TrimSuffix(domain, ".")
 	}
 	parsedFrom, err := mail.ParseAddress(cfg.FromAddress)
 	if err != nil {

checker/definition.go

@@ -35,19 +35,6 @@ func Definition() *sdk.CheckerDefinition {
 				},
 			},
 			UserOpts: []sdk.CheckerOptionDocumentation{
-				{
-					Id:          "happydeliver_url",
-					Type:        "string",
-					Label:       "happyDeliver instance URL (override)",
-					Description: "Override the operator-provided happyDeliver URL.",
-				},
-				{
-					Id:          "happydeliver_token",
-					Type:        "string",
-					Label:       "happyDeliver API token (override)",
-					Description: "Override the operator-provided happyDeliver token.",
-					Secret:      true,
-				},
 				{
 					Id:          "smtp_host",
 					Type:        "string",
@@ -84,13 +71,6 @@ func Definition() *sdk.CheckerDefinition {
 					Choices:     []string{"starttls", "tls", "none"},
 					Default:     "starttls",
 				},
-				{
-					Id:          "from_address",
-					Type:        "string",
-					Label:       "From address",
-					Description: "Address used in the From header of the test email. Must be in the tested domain.",
-					Required:    true,
-				},
 				{
 					Id:          "subject_override",
 					Type:        "string",
@@ -132,6 +112,12 @@ func Definition() *sdk.CheckerDefinition {
 					Label:    "Domain name",
 					AutoFill: sdk.AutoFillDomainName,
 				},
+				{
+					Id:          "from_address",
+					Type:        "string",
+					Label:       "From address",
+					Description: "Address used in the From header of the test email. Must be in the tested domain. Defaults to no-reply@<domain>.",
+				},
 			},
 		},
 		Rules: []sdk.CheckRule{

checker/provider.go

@@ -2,6 +2,9 @@ package checker
 
 import (
 	"encoding/json"
+	"fmt"
+	"net/mail"
+	"strings"
 	"time"
 
 	sdk "git.happydns.org/checker-sdk-go/checker"
@@ -21,6 +24,27 @@ func (p *happyDeliverProvider) Definition() *sdk.CheckerDefinition {
 	return Definition()
 }
 
+// ValidateOptions runs on the host before Collect so bad options are rejected
+// up-front rather than surfacing as a runtime SMTP failure.
+func (p *happyDeliverProvider) ValidateOptions(opts sdk.CheckerOptions) error {
+	raw, ok := opts["from_address"]
+	if !ok {
+		return nil
+	}
+	s, ok := raw.(string)
+	if !ok {
+		return fmt.Errorf("from_address must be a string")
+	}
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return nil
+	}
+	if _, err := mail.ParseAddress(s); err != nil {
+		return fmt.Errorf("invalid from_address: %w", err)
+	}
+	return nil
+}
+
 func (p *happyDeliverProvider) ExtractMetrics(ctx sdk.ReportContext, collectedAt time.Time) ([]sdk.CheckMetric, error) {
 	var data HappyDeliverData
 	if err := json.Unmarshal(ctx.Data(), &data); err != nil {

checker/provider_test.go

@@ -86,8 +86,8 @@ func TestDefinitionShape(t *testing.T) {
 	if def.ID != "happydeliver" {
 		t.Errorf("ID = %q", def.ID)
 	}
-	if !def.Availability.ApplyToDomain {
-		t.Error("should apply to domain")
+	if !def.Availability.ApplyToService {
+		t.Error("should apply to service")
 	}
 	if !def.HasMetrics {
 		t.Error("HasMetrics should be true")