通过Virtualmin的Usermin面板发送长邮件被拒收，短邮件正常的问题排查求助

大家好，我碰到一个很奇怪的邮件发送问题，想请各位帮忙一起分析排查：

我用Virtualmin旗下的Usermin面板发送邮件，不管是从xx@domain.com还是其他邮箱账号出发，短内容的邮件（比如标题和正文都只有"hi"）都能正常投递到Gmail、Hotmail等平台的收件箱；但只要发送长文本内容的邮件，就会被Gmail直接退回，退回的错误信息如下：

xx@gmail.com: host gmail-smtp-in.l.google.com[xx.xx.153.27] said:
550-5.7.1 [xx.xx.xx.167 12] Gmail has detected that this message is
550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to
Gmail, 550-5.7.1 this message has been blocked. Please visit 550-5.7.1
support.google.com/mail/?p=UnsolicitedMessageError for more 550
5.7.1 information. gz13-20020a170906f2cd00b00a0e79089b8fsi528438ejb.874 -
gsmtp (in reply to end of DATA command)

我已经确认过域名的SPF和DMARC记录都配置正确，而且用mail-tester.com测试邮件评分是满分10/10，短邮件的投递也完全正常，唯独长邮件会触发Gmail的垃圾邮件拦截机制。

给大家举两个具体的例子：

• 能正常收到的邮件：
  标题: hi
  内容: hi
• 被拒收的邮件：
  标题: hi
  内容: Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris pellentesque auctor consectetur. Phasellus quis ipsum consequat, ultrices est eget, mollis magna. Maecenas porttitor enim vel laoreet vestibulum. Phasellus tincidunt mi in erat efficitur, a maximus est vestibulum. Etiam faucibus purus quis euismod accumsan. Vestibulum placerat mollis erat vitae elementum. Sed id lacinia augue. Sed venenatis, tellus in elementum mollis, eros diam maximus arcu, eu bibendum risus justo ac nunc. Phasellus suscipit ex et augue ultricies, mollis tincidunt est consequat. Morbi id sollicitudin purus, sed consequat orci. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia curae; Sed tincidunt nunc et pretium fringilla. Suspendisse sollicitudin, lectus nec fringilla iaculis, metus turpis maximus massa, vitae elementum tellus nisl eleifend massa. Morbi finibus, velit nec eleifend maximus, nisl nunc elementum diam, hendrerit porttitor nisl ipsum at purus. Integer convallis rhoncus dolor ac condimentum. Sed suscipit vitae tellus eget gravida.

有没有朋友遇到过类似的情况？或者能给我一些具体的排查方向吗？非常感谢！

备注：内容来源于stack exchange，提问作者Khodour.F