refactor: use built in smtp library (close #34)

common/email.go

@@ -1,14 +1,20 @@
 package common
 
-import "gopkg.in/gomail.v2"
+import (
+	"fmt"
+	"net/smtp"
+	"strings"
+)
 
 func SendEmail(subject string, receiver string, content string) error {
-	m := gomail.NewMessage()
-	m.SetHeader("From", SMTPAccount)
-	m.SetHeader("To", receiver)
-	m.SetHeader("Subject", subject)
-	m.SetBody("text/html", content)
-	d := gomail.NewDialer(SMTPServer, SMTPPort, SMTPAccount, SMTPToken)
-	err := d.DialAndSend(m)
+	mail := []byte(fmt.Sprintf("To: %s\r\n"+
+		"From: %s<%s>\r\n"+
+		"Subject: %s\r\n"+
+		"Content-Type: text/html; charset=UTF-8\r\n\r\n%s\r\n",
+		receiver, SystemName, SMTPAccount, subject, content))
+	auth := smtp.PlainAuth("", SMTPAccount, SMTPToken, SMTPServer)
+	addr := fmt.Sprintf("%s:%d", SMTPServer, SMTPPort)
+	to := strings.Split(receiver, ";")
+	err := smtp.SendMail(addr, auth, SMTPAccount, to, mail)
 	return err
 }