Why not set your webserver to send it directly to xmail.com?

Having a second mail server seems redundant.

Anyway, set your Mercury relay server to be called relay.xmail.com or such and set a Connection Control entry in the MercS config to allow relaying for your webserver.

Hey all, I'm having a bit of trouble with a rather odd setup situation.
Let me preface this in saying I'm not all too familiar with Mercury, working with it for about a week now. I am however familiar with SMTP protocols and networking. 

So we've got a Wordpress site setup on a dedicated webserver, and when somebody fills out a form(such as contact) in Wordpress, the email is passed along to Mercury which will then send it to a dedicated Email server on the same domain MX(different IP); and I'm not really sure how to do this properly. I've closed off port 25 for external access(on the Mercury server), as only the local PHP/Wordpress is going to be sending mails with Mercury, and it only needs to send, not receive. I need outgoing mails to go to the MX address, but I need the emails to appear as though their coming from the same domain name.

Lets say the domain name is xmail.com, 60.*.*.* is the webserver with Apache/PHP/Wordpress/Mercury, and 201.*.*.* is the email server on the MX. When Mercury sends an email to the 201 email server, those emails need to appear from [x]@xmail.com, so that the email server will accept it.

Most of my problems are arising from similar domains- my mind is a bit mudded so I can't really give details but for instance when I set the Core internet name to xmail.com, it will send the emails but if there's an error in sending an email, Mercury will then generate the error notification and send it to admin/postmaster @ the MX email server instead of the local admin/postmaster, meaning if that notification fails then it will generate another notification, and another, and loop. Now, if I had xmail.com set as a local address, then it's just trying to connect to itself every time it's sending an email to that address.

So for a summary of what I want to happen is.. PHP --> Mercury(60.*.*.*:80 @ xmail.com server) --> Sends mail FROM [x]@xmail.com TO 201.*.*.*:25 @ xmail.com server.

Just blah blah blah, things like that, the configurations are starting to blur together for me so if anybody could provide guidance for me it would be appreciated.
If you need more details or a different explanation please ask. Thanks.