FastMail 的好处之一就是你真的可以找到客服,而且客服真的可以帮你解决问题。
我跟客服问过这个问题,客服解释是,通过邮件客户端发送的时候,会带上 ISP 的 IP ,这个 IP 被 QQ 邮箱的服务器 block 了,所以会有 550 IP Frequency Limited 错误。换用官方的 Web 界面发就好了。客服原文如下:
OK, you appear to be using an email client to send your emails.
When you send emails using an external client, it would include your ISP IP in the header of the email. So, if that IP is blacklisted, which happens quite commonly with ISPs, the emails would be filtered as spam.
To confirm on this, can you please try sending the email from the webinterface (
www.fastmail.com)? See if the email is then being listed as spam.