邮件HTML真是难搞，这么多年不做新的语法支持。删删改改，最终写出了一套满意的，必须使用内联CSS样式和表格布局，旧属性旧语法真的很心累。
验证码邮件模板
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="zh-CN">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>{{.Title}}</title>
</head>
<body style="margin: 0; padding: 0;">
<table border="0" cellpadding="0" cellspacing="0" width="100%"
style="background-image: url('https://testingcf.jsdelivr.net/gh/4444TENSEI/CDN@master/img/background/pt/003.webp'); background-size: cover; background-position: center; background-repeat: no-repeat; height: 960px;">
<tr>
<td>
<table border="0" cellpadding="0" cellspacing="0" width="600" align="center">
<tr>
<td align="center">
<!-- 这里设置卡片宽度 -->
<div style="max-width: 74%;width: auto; background-color: rgba(100, 120, 140, 0.6); padding: 36px 0; border-radius: 60px; text-align: center;
-webkit-backdrop-filter: blur(10px);
backdrop-filter: blur(10px);">
<p
style="font-size: 36px; line-height: 1;margin:10px 0 32px 0; color: rgb(255, 255, 255); background-color: rgba(255, 255, 255, 0);white-space: nowrap;">
您的{{.Purpose}}验证码是:
</p>
<p
style="font-size: 80px; line-height: 1;margin:0; color: rgb(255, 255, 255); font-weight: 1000; background-color: rgba(255, 255, 255, 0);white-space: nowrap;">
{{.Captcha}}
</p>
</div>
</td>
</tr>
</table>
</td>
</tr>
</table>
</body>
</html>