关于邮件发送失败：Delivery Status Notification (Delay)

Hello，

现在有些用户报告有时候通过gmail发送邮件给exchange邮件用户失败，退信内容如下，不知大家有何高见？非常感谢！

This is an automatically generated Delivery Status Notification

THIS IS A WARNING MESSAGE ONLY.

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

Delivery to the following recipient has been delayed:

    who@xzy.com

Message will be retried for 1 more day(s)

Technical details of temporary failure:
Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 421 421 4.4.1 Connection timed out (state 18).