向Gmail邮箱发信时，提示如下退信：
host

    gmail-smtp-in.l.google.com[64.233.190.27] said: 550-5.7.26 Your email has

    been blocked because the sender is unauthenticated. 550-5.7.26 Gmail

    requires all senders to authenticate with either SPF or DKIM. 550-5.7.26

    550-5.7.26  Authentication results: 550-5.7.26  DKIM = did not pass

    550-5.7.26  SPF [XXXXXX.com] with ip: [200.55.240.110] = did not pass

    550-5.7.26  550-5.7.26  For instructions on setting up authentication, go

to 550 5.7.26 https://support.google.com/mail/answer/81126#authentication

    586e51a60fabf-2b9548cb6a2si860921fac.62 - gsmtp (in reply to end of DATA

    command)

退信原因：
1. 邮箱没有配置SPF或者DKIM协议
2. 域名解析平台不支持SPF或者DKIM解析

解决方案（任意选择其中一种）：
1. 为邮箱配置SPF或者DKIM协议
2. 使用知名的域名解析平台，更加稳定，以及更高的邮件通达率
3. 联系收件方，将发件方邮件地址添加进白名单

Reason for return:

1. Email does not have SPF or DKIM protocol configured

2. The domain name resolution platform does not support SPF or DKIM resolution

Solution (choose any one):

1. Configure SPF or DKIM protocol for email

2. Use a well-known domain name resolution platform for greater stability and higher email accessibility

3. Contact the recipient and add the sender's email address to the whitelist