Question : Mail not being sent

I hav a user trying to sent email to a client and receiving this error

An error occurred while trying to deliver this message to the recipient's e-mail address. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message, or provide the following diagnostic text to your system administrator.

Answer : Mail not being sent

Please have a read of my article about problems sending mail to one or more domains:

http://www.experts-exchange.com/articles/Software/Server_Software/Email_Servers/Problems-sending-mail-to-one-or-more-external-domains.html