Send mails from a service account via Gmail API

main.go

package sendMail

import (
	"bytes"
	"context"
	"encoding/base64"
	"encoding/json"
	"fmt"

	"cloud.google.com/go/pubsub"
	"google.golang.org/api/gmail/v1"
	"google.golang.org/api/impersonate"
	"google.golang.org/api/option"
)

type Message struct {
	ServiceAccount   string `json:"service_account"`
	UserEmail        string `json:"user_email"`
	DestinationEmail string `json:"destination_email"`
}

func SendMail(ctx context.Context, m *pubsub.Message) error {
	var message Message
	json.Unmarshal(m.Data, &message)

	ts, err := impersonate.CredentialsTokenSource(ctx, impersonate.CredentialsConfig{
		TargetPrincipal: message.ServiceAccount,
		Scopes:          []string{gmail.GmailSendScope},
		Subject:         message.UserEmail,
	})
	if err != nil {
		return fmt.Errorf("failed to create impersonated token source: %v", err)
	}

	srv, err := gmail.NewService(ctx, option.WithTokenSource(ts))
	if err != nil {
		return fmt.Errorf("unable to create Gmail client: %v", err)
	}

	from := message.UserEmail
	to := message.DestinationEmail

	subject := "Test Mail from Gmail API via Impersonated Credentials (Cloud Functions Event Trigger)"
	body := "This is a test mail from Gmail API via Impersonated Credentials (Cloud Functions Event Trigger)."

	msg := createMessage(from, to, subject, body)

	_, err = srv.Users.Messages.Send("me", msg).Do()
	if err != nil {
		return fmt.Errorf("unable to send message: %v", err)
	}

	fmt.Println("Email sent successfully.")
	return nil
}

func createMessage(from, to, subject, body string) *gmail.Message {
	header := make(map[string]string)
	header["From"] = from
	header["To"] = to
	header["Subject"] = subject
	header["Content-Type"] = "text/plain; charset=\"UTF-8\""

	var msg bytes.Buffer
	for k, v := range header {
		msg.WriteString(fmt.Sprintf("%s: %s\r\n", k, v))
	}
	msg.WriteString("\r\n")
	msg.WriteString(body)

	return &gmail.Message{
		Raw: base64.RawURLEncoding.EncodeToString(msg.Bytes()),
	}
}