Could you please elaborate , is this a fresh installation or migration from an old server . You can send email because smtp service is running on your lotus domino server and if port 25 is open in your firewall it will allow you to send any email from any smtp server even a simple vista smtp server.
There are several possible causes of the problem.
First review the basic requrements for smtp colaboration:
1. Fully qualified domain name for your Smtp server.
2. Smtp relay host ( if you are relaying your mx record from ISP, you should configure this on your Domino configuration documents smtp relay host)
3. Check what Gabe9527 mentioned, If you have a firewall, definetely you are forwarding your smtp public ip to you locally configured LAN ip of the mail server. Check if smtp traffic is properly forwarded to the lan IP of your mail server and you should allow smtp traffic in the packet filter.