feat: harden SMTP transport

Add explicit TLS modes, context-aware delivery, and typed transport errors.
Preserve raw MIME messages and cover the new delivery paths with local SMTP tests.

Makefile

@@ -1,13 +1,34 @@
-COVERAGE_DIR=coverage
+COVERAGE_DIR := coverage
 
-lint:
+.PHONY: help fmt fmt-check vet lint test test-race test-coverage check clean
+
+help: ## Show available targets
+	@awk 'BEGIN {FS = ":.*?## "} /^[a-zA-Z_-]+:.*?## / {printf "  %-15s %s\n", $$1, $$2}' $(MAKEFILE_LIST)
+
+fmt: ## Format Go source files
+	gofmt -w .
+
+fmt-check: ## Check Go source formatting
+	@test -z "$$(gofmt -l .)"
+
+vet: ## Run Go static analysis
+	go vet ./...
+
+lint: ## Run golangci-lint
 	golangci-lint run
-goreportcard:
-	goreportcard-cli -v
-test:
-	go test ./... 
-test-coverage:
-	rm -rf ${COVERAGE_DIR}
-	mkdir ${COVERAGE_DIR}
-	go test -v -coverprofile ${COVERAGE_DIR}/cover.out ./...
-	go tool cover -html ${COVERAGE_DIR}/cover.out -o ${COVERAGE_DIR}/cover.html
+
+test: ## Run tests
+	go test ./... -count=1
+
+test-race: ## Run tests with the race detector
+	go test -race ./... -count=1
+
+test-coverage: ## Generate an HTML coverage report
+	mkdir -p $(COVERAGE_DIR)
+	go test ./... -count=1 -coverprofile=$(COVERAGE_DIR)/cover.out
+	go tool cover -html=$(COVERAGE_DIR)/cover.out -o $(COVERAGE_DIR)/cover.html
+
+check: fmt-check vet test test-race lint ## Run all validation checks
+
+clean: ## Remove generated coverage files
+	rm -rf $(COVERAGE_DIR)