use with ... for smtp

app/email_utils.py

@@ -1297,8 +1297,14 @@ def sl_sendmail(
         return
 
     try:
-        # to avoid creating SMTP server every time
-        smtp = get_smtp_server()
+        if POSTFIX_SUBMISSION_TLS:
+            smtp_port = 587
+        else:
+            smtp_port = POSTFIX_PORT
+
+        with SMTP(POSTFIX_SERVER, smtp_port) as smtp:
+            if POSTFIX_SUBMISSION_TLS:
+                smtp.starttls()
 
             # smtp.send_message has UnicodeEncodeError
             # encode message raw directly instead