diff --git a/manual/config/sending_email.md b/manual/config/sending_email.md
index 1670fe6a..8b159ee8 100644
--- a/manual/config/sending_email.md
+++ b/manual/config/sending_email.md
@@ -65,6 +65,28 @@ interval = 30m
 
 ```
 
+## Add S/MIME signature to email
+
+If you want the email signed by S/MIME, please add the config in `seahub_settings.py`
+
+
+```python
+ENABLE_SMIME = True
+SMIME_CERTS_DIR = /opt/seafile/seahub-data/smime-certs # including cert.pem and private_key.pem
+
+```
+The certificate can be generated by the openssl command, or you can apply to the certificate manufacturer, it is up to you.
+For example, generate the certs using the following command:
+
+```
+mkdir -p /opt/seafile/seahub-data/smime-certs
+cd /opt/seafile/seahub-data/smime-certs
+openssl req -x509 -newkey rsa:4096 -keyout private_key.pem -outform PEM -out cert.pem -days 3650 -nodes
+```
+!!! tip
+    Some email clients may not verify the email signed by certs generated by command line. So it's better to apply the certs from a manufacture
+
+
 ## Customize email messages
 
 The simplest way to customize the email messages is setting the `SITE_NAME` variable in `seahub_settings.py`. If it is not enough for your case, you can customize the email templates.