帮忙翻译下This is the mail system at host gw171.tom.com.I'm sorry

帮忙翻译下
This is the mail system at host gw171.tom.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
: host 172.24.192.91[172.24.192.91] said: 550
: Invalid User (in reply to RCPT TO command)
User (in reply to RCPT TO command)