We are seeing a problem where mails (registration notices etc) are not making it out to users with email addresses that are not in the domain of the server we are setting up. For instance our site is www.sluse.com and if I create a new user with an email @sluse.com it will send the email, but if its another address it doesnt send it out.

We are using SENDMAIL option in XOOPS and the box is a Red Hat Enterprise Linux 4.0 AS server. We own the entire box and are not actively (atleast on purpose) blocking any emails etc. This looks liek it may be some issue with Linux setup rather then XOOPS, but I am not sure how to fix this issue. Anyone know what we can do so all emails get sent out?