only allow mails to addresses that contain the mail server domain

there were some mails coming in that was not for any temp mail addr,
it was like spam@otherdomain.com, but still reaching the database. This
should prevent this.

pkg/mail_server/main.go

@@ -8,6 +8,7 @@ import (
 	"log"
 	"os"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/GRFreire/nthmail/pkg/mail_utils"
@@ -16,7 +17,8 @@ import (
 )
 
 type Backend struct {
-	db *sql.DB
+	db     *sql.DB
+	domain string
 }
 
 func (backend *Backend) NewSession(c *smtp.Conn) (smtp.Session, error) {
@@ -26,7 +28,8 @@ func (backend *Backend) NewSession(c *smtp.Conn) (smtp.Session, error) {
 	}
 
 	return &Session{
-		tx: tx,
+		tx:     tx,
+		domain: backend.domain,
 	}, nil
 }
 
@@ -34,6 +37,16 @@ type Session struct {
 	tx         *sql.Tx
 	from, rcpt string
 	arrived_at int64
+	domain     string
+}
+
+func get_addr_domain(addr string) string {
+	index := strings.Index(addr, "@")
+	if index < 0 {
+		return ""
+	}
+
+	return addr[index+1:]
 }
 
 func (session *Session) AuthPlain(username, password string) error {
@@ -48,6 +61,9 @@ func (session *Session) Mail(from string, opts *smtp.MailOptions) error {
 }
 
 func (session *Session) Rcpt(to string, opts *smtp.RcptOptions) error {
+	if get_addr_domain(to) != session.domain {
+		return errors.New("To addr domain is not available in this server")
+	}
 	session.rcpt = to
 
 	return nil
@@ -55,33 +71,38 @@ func (session *Session) Rcpt(to string, opts *smtp.RcptOptions) error {
 
 func (session *Session) Data(reader io.Reader) error {
 	defer session.tx.Rollback()
-	if bytes, err := io.ReadAll(reader); err != nil {
+
+	bytes, err := io.ReadAll(reader)
+	if err != nil {
 		return err
-	} else {
-
-		stmt, err := session.tx.Prepare("INSERT INTO mails (arrived_at, rcpt_addr, from_addr, subject, data) VALUES (?, ?, ?, ?, ?)")
-		if err != nil {
-			println(err)
-			return err
-		}
-		defer stmt.Close()
-
-		mail_obj, err := mail_utils.Parse_mail(bytes, true)
-		if err != nil {
-			return err
-		}
-
-		_, err = stmt.Exec(session.arrived_at, session.rcpt, mail_obj.From, mail_obj.Subject, bytes)
-		if err != nil {
-			return err
-		}
-
-		err = session.tx.Commit()
-		if err != nil {
-			return err
-		}
-
 	}
+
+	mail_obj, err := mail_utils.Parse_mail(bytes, true)
+	if err != nil {
+		return err
+	}
+
+	if get_addr_domain(mail_obj.To) != session.domain {
+		return errors.New("To addr domain is not available in this server")
+	}
+
+	stmt, err := session.tx.Prepare("INSERT INTO mails (arrived_at, rcpt_addr, from_addr, subject, data) VALUES (?, ?, ?, ?, ?)")
+	if err != nil {
+		println(err)
+		return err
+	}
+	defer stmt.Close()
+
+	_, err = stmt.Exec(session.arrived_at, session.rcpt, mail_obj.From, mail_obj.Subject, bytes)
+	if err != nil {
+		return err
+	}
+
+	err = session.tx.Commit()
+	if err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -92,12 +113,6 @@ func (session *Session) Logout() error {
 }
 
 func Start(db *sql.DB) error {
-	backend := &Backend{
-		db: db,
-	}
-
-	server := smtp.NewServer(backend)
-
 	domain, exists := os.LookupEnv("MAIL_SERVER_DOMAIN")
 	if !exists {
 		domain = "localhost"
@@ -115,6 +130,13 @@ func Start(db *sql.DB) error {
 		port = 1025
 	}
 
+	backend := &Backend{
+		db:     db,
+		domain: domain,
+	}
+
+	server := smtp.NewServer(backend)
+
 	server.Addr = fmt.Sprintf(":%d", port)
 	server.Domain = domain
 	server.WriteTimeout = 60 * time.Second