Two Servers and Two Sets of Email Addresses

[yurtboy1]

Hello,
We are hosting a clients web site but not email.
Yet we need one email to reside on our server as a catch all since their host does not allow this. It is key for our mass email software (civicrm.org)

On their serves I will setup
MX 30 theirserver
MX 50 our ispconfig server

On our server under MX I was going to enter
Domain Settings
-MX
--Hostname blank
--Prio 50
--MailServer Theirs

-MX
--Hostname blank
--Prio 90
--MailServer Ours

This way if I send them an email from our server they still get it on their server.
And then if the catchall email on their server gets and email it will forward it to our server which should allow it in.

Also I am not sure how the External and Local Email servers in ISPCONFIG effect all this.
Right now even with the above setup it seems I need to set all of these to External Email Server?

Any advices would be appreciated