C# .NET - 554 relay access denied exchange

Hi,
   we get error "554 <abc@abc.com>: relay access denied" only with a few mail recipients. My configuration: Exchange 2007 SP1

Please help me for this issue

As explained here - http://forums.msexchange.org/m_1800494984/tm.htm , Relaying means an attempt to send a message to an SMTP server not acting as the authority for the recipient address.

Check the above link for tips on resolving the issue.