Thanks for the reply.
I am able to ping the box, and I can reach the box's default apache page. Also, the mail is getting to the box. The log shows sendmail makes 10-15 attempts to deliver the message and returns errors with 'host unknown' and 'host not found'
I tried several different test accounts (all in the popusers group.) In each case the message goes to the root mailbox as unresolved. I also sent a message from the test server itself and it also went into the root mailbox. Thanks.