Modify rcpthost to allow a certain domain

I am not able to send any email to a client. When I try to do that my mail bounce back. The mail contains a error description which says that it is related to some SMTP communication problem that lies with the recipient mail server. It is a bit annoying and I need help here to modify the rcpthosts. How can I do that. I am not aware about the exact process. I hope someone can give me more guideline on the same.

I think the problem lies with the service provider. I do not think you will need to modify the mail server configuration. You just need to check out the mail settings and then try back. The rcpthosts contains the list of approved and unapproved recipient domain host. I think you can find out more detail information on the web related to that and you can find more related information.