add config check

internal/configuration/configuration.go

@@ -48,8 +48,38 @@ func (c Config) LoadFromDisk() error {
 		return fmt.Errorf("failed to unmarshal config file: %w", err)
 	}
 
+	c.checkConfig()
+
 	slog.Debug("successfully read config file", "path", ConfigFile)
 
 	return nil
 
 }
+
+func (c Config) checkConfig() {
+	failed := false
+	if c.ApiEndpoint == "" {
+		slog.Error("[CONFIG] api_endpoint must not be empty")
+		failed = true
+	}
+
+	if c.ApiKey == "" {
+		slog.Error("[CONFIG] api_key must not be empty")
+		failed = true
+	}
+
+	if c.AdminEmail == "" {
+		slog.Error("[CONFIG] admin_email must not be empty")
+		failed = true
+	}
+
+	if len(c.MailPrefixes) < 1 {
+		slog.Error("[CONFIG] mail_prefixes must have at least one entry")
+		failed = true
+	}
+
+	if failed {
+		slog.Error("[CONFIG] Application can't start until above errors are fixed.")
+		os.Exit(1)
+	}
+}