以下是之前回答中的几个要点:

Most important: Does the sender address ("From") belong to a domain that runs on the server you send the E-Mail from? If not, make it so. Never use sender addresses like xxx@gmail.com. User reply-to if you need replies to arrive at a different address. Is your server on a blacklist (e.g. check IP on spamhaus.org)? This is a possibility when you're on shared hosting when neighbours behave badly. Are mails filtered by a spam filter? Open an account with a freemailer that has a spam folder and find out. Also, try sending mail to an address without any spam filtering at all. Do you possibly need the fifth parameter "-f" of mail() to add a sender address? (See mail() command in the PHP manual) If you have access to log files, check those, of course. Do you check the "from:" address for possible bounce mails ("Returned to sender")? You can also set up a separate "errors-to" address.

在尽可能多的主要电子邮件提供商(gmail/yahoo/hotmail/aol/etc)上注册一个帐户。如果你改变了你的电子邮件，无论是重大的措辞，改变了发送电子邮件的代码，改变了你的电子邮件服务器，等等，一定要向你所有的账户发送测试邮件，并验证它们没有被标记为垃圾邮件。

雅虎使用一种称为发件人ID的方法，可以在SPF设置向导中配置并输入到您的DNS中。另外，对于Exchange, Hotmail, AOL, Yahoo和其他公司来说，重要的一点是为你的域名提供反向DNS。这将解决大部分问题。但是，您永远无法阻止某人故意阻止您的或自定义规则。

我还要补充说:

提供真正的取消订阅点击“取消订阅”。我见过真正的通讯提供一个虚拟的退订链接，点击显示“已成功退订”，但我仍然会收到进一步的通讯。

你能做的最重要的事情是确保你要发送电子邮件的人在收到你的电子邮件时不太可能点击“垃圾邮件”按钮。所以，请遵循以下经验法则:

Make sure you have permission from the people you are sending email to. Don't ever send email to someone who did not request it from you. Clearly identify who you are right at the top of each message, and why the person is receiving the email. At least once a month, send out a reminder email to people on your list (if you are running a list), forcing them to opt back in to the list in order to keep receiving communications from you. Yes, this will mean your list gets shorter over time, but the up-side is that the people on your list are "bought in" and will be less likely to flag your email. Keep your content highly relevant and useful. Give people an easy way to opt out of further communications. Use an email sending service like SendGrid that works hard to maintain a good IP reputation. Avoid using short links - these are often blacklisted.

遵循这些经验法则将大有裨益。

在过去的工作中，我在很多网站上都遇到过同样的问题。确保用户收到邮件的唯一保证方法是建议用户将你添加到安全列表中。其他任何方法都只能帮助解决这个问题，而且不能保证。