docs: update README

README.md

@@ -334,7 +334,9 @@ Use [healthcheck.io](https://healthchecks.io/) url or similar cron monitoring to
 
 #### MAIL_SMTP_ENABLE
 
-The tool uses [heirloom-mailx](https://www.systutorials.com/docs/linux/man/1-heirloom-mailx/) to send mail.
+Starting from v1.19.0, we will be using [`s-nail`](https://www.sdaoden.eu/code-nail.html) instead of [`heirloom-mailx`](https://www.systutorials.com/docs/linux/man/1-heirloom-mailx/) to send emails.
+
+Please note that `heirloom-mailx` is a stub for `s-nail`, and most of its functionality is compatible. Therefore, you may not need to modify any environment variables for this change.
 
 Default: `FALSE`
 
@@ -358,8 +360,6 @@ During testing, we will add the `-v` option to display detailed information.
 -S from=<my-email-address>
 ```
 
-For more information, refer to [here](https://www.systutorials.com/sending-email-from-mailx-command-in-linux-using-gmails-smtp/).
-
 #### MAIL_TO
 
 This specifies the recipient of the notification email.

README_zh.md

@@ -332,7 +332,9 @@ Rclone 全局参数，详见 [flags](https://rclone.org/flags/)。
 
 #### MAIL_SMTP_ENABLE
 
-本工具使用 [heirloom-mailx](https://www.systutorials.com/docs/linux/man/1-heirloom-mailx/) 发送邮件。
+从 v1.19.0 开始，本工具使用 [`s-nail`](https://www.sdaoden.eu/code-nail.html) 代替 [`heirloom-mailx`](https://www.systutorials.com/docs/linux/man/1-heirloom-mailx/) 发送邮件。
+
+请注意，`heirloom-mailx` 是 `s-nail` 的基础，它们大部分功能是兼容的。因此你可能不需要为这个改变修改任何环境变量。
 
 默认值：`FALSE`
 
@@ -356,8 +358,6 @@ Rclone 全局参数，详见 [flags](https://rclone.org/flags/)。
 -S from=<my-email-address>
 ```
 
-在 [这里](https://www.systutorials.com/sending-email-from-mailx-command-in-linux-using-gmails-smtp/) 查看更多配置说明。
-
 #### MAIL_TO
 
 指定通知邮件的接收者。