SMTP issues in SBS

hi,
i have recentely install Windows Small Business Server 2008,it give an error :" There was a SMTP communication problem with the recipient's email server. Please contact your system administrator."i think the issue is that i can send maill , for instence to Gmail , however when i reply to the same mail i send the got it in Gmail and i get this error.Any recommendations and suggestions are appreciated.

hi,
this problem occur if The IP address does not match the domain name that is used in the return address of the e-mail message.
A pointer (PTR) record does not exist or is not valid for the source SMTP server's IP address.

Use the NSLookup utility to make sure that the MX and A records of the Exchange Server domain on the Internet are correct.
If the problem is related to the incorrect configuration of an echo firewall, then correct the configuration of the firewall.

hi,
Since you are able to reach some recipients successfully, the problem is likely the receiving server. The sever is busy, internet connection to server is busy. Or server is blocking your address.