fix: allow setting the mailer service headers as strings

internal/conf/configuration.go

@@ -403,18 +403,18 @@ type MailerConfiguration struct {
 	// EXPERIMENTAL: May be removed in a future release.
 	EmailValidationExtended       bool   `json:"email_validation_extended" split_words:"true" default:"false"`
 	EmailValidationServiceURL     string `json:"email_validation_service_url" split_words:"true"`
-	EmailValidationServiceHeaders string `json:"email_validation_service_key" split_words:"true"`
+	EmailValidationServiceHeaders string `json:"email_validation_service_headers" split_words:"true"`
 
-	serviceHeaders map[string][]string `json:"-"`
+	serviceHeaders map[string]any `json:"-"`
 }
 
 func (c *MailerConfiguration) Validate() error {
-	headers := make(map[string][]string)
+	headers := make(map[string]any)
 
 	if c.EmailValidationServiceHeaders != "" {
 		err := json.Unmarshal([]byte(c.EmailValidationServiceHeaders), &headers)
 		if err != nil {
-			return fmt.Errorf("conf: SMTP headers not a map[string][]string format: %w", err)
+			return fmt.Errorf("conf: mailer validation headers not a map[string][]string format: %w", err)
 		}
 	}
 
@@ -424,7 +424,7 @@ func (c *MailerConfiguration) Validate() error {
 	return nil
 }
 
-func (c *MailerConfiguration) GetEmailValidationServiceHeaders() map[string][]string {
+func (c *MailerConfiguration) GetEmailValidationServiceHeaders() map[string]any {
 	return c.serviceHeaders
 }
 

internal/mailer/validate.go

@@ -79,7 +79,7 @@ var (
 type EmailValidator struct {
 	extended       bool
 	serviceURL     string
-	serviceHeaders map[string][]string
+	serviceHeaders map[string]any
 }
 
 func newEmailValidator(mc conf.MailerConfiguration) *EmailValidator {
@@ -197,14 +197,21 @@ func (ev *EmailValidator) validateService(ctx context.Context, email string) err
 	}
 
 	rdr := bytes.NewReader(reqData)
-	req, err := http.NewRequestWithContext(ctx, "GET", ev.serviceURL, rdr)
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, ev.serviceURL, rdr)
 	if err != nil {
 		return nil
 	}
 	req.Header.Set("Content-Type", "application/json")
 	for name, vals := range ev.serviceHeaders {
-		for _, val := range vals {
-			req.Header.Set(name, val)
+		switch v := vals.(type) {
+		case string:
+			req.Header.Set(name, v)
+		default:
+			b, err := json.Marshal(v)
+			if err != nil {
+				return err
+			}
+			req.Header.Set(name, string(b))
 		}
 	}