How to ensure Exchange 2003 responding to greylistings correctly.?

Our emails are being rejected to ???info@ourclient.com???. this is happening only to them as far as i know. their IT guy has advised me that the issue is from our side. according to him our email server isn???t responding to greylistings correctly.

how to ensure that messages are attempted to be resent ? where can i find these settings ? we are using SBS exchange 2003. your help much appreciated. thanks