http://www.bynari.net/how_do_i_configure_postfix_to_route_mail_to_an_internal_domain/Apols but it is my current love affair with postfix. You can actually bypass dns and mx and just point to an IP.
Postfix for me is the swiss army smtp server. So if it was about using Zentyal as a gateway then the only consideration for me was external imap / pop requests.
Don't know why but I made the assumption that you work in the manner for want of a better description "Call Center", loads of domains rather than clients. All the mail needs routing to internal servers where pop / imap whatever is done internally.
Christian has put a few solid solutions down but you could also use a single IP with Zentyal as a gateway with postfix being the piggy in the middle acting purely as an incoming router and outgoing relay.
In the post you say you have existing mail servers so I am also presuming you have quite a bit of infrastructure and you are think of using zentyal as purely a advanced gateway server?
I have done the rounds with firewall routers from pfsense to endian and even ipcop I always return to one place and that is Zentyal.
My personal opinion is there are two leading gateway servers and obviously I say Zentyal. I am a fan of opensource so I will say ClearOS is a great piece of software.
I have strong reasons for this and feel that sometimes CentOS is a little slow with releases and if your a Debian / Ubuntu person then thats your choice Zentyal.
I always found the other firewalls superb bits of kit but always missed just one key item.
If your handling quite a bit of traffic then I would definitely get in touch with zentyal.
Talking about swiss army SMB servers and the use of enterprise grade services like postfix is that I often have customisation needs. Zentyal allows you to assemble several forms of infrastructure at the press of a button. Its this instant base that you can use as a template to add your too your solution for your particular needs.
It gives you a simple common interface to quickly create a server. As far as I remember even if the other firewalls did have scripting possibilities I am finding Zentyal easier.
Zentyal is a superb gateway server if I was going to use :-
Firewall
IDS\IPS
Trafic management
Traffic balancing
Wan Failover.
RAID
Events (Notifications)
Proxy
VPN
DHCP
DNS
Portforwarding
NAT/SNAT
UPS
And the ones I forgot but that can be assembled quickly and maybe either just add postfix manually or customise the mail offering.
The Zentyal support team are a no brainer on this sort of stuff as with a good combination of design and hardware it can scale well.
PS
These are pure guesses to your operation but its more of an example of using zentyal. Its really good as a platform for various solutions, with a bit of scripting you can add custom functions. The example had an LDAP script which with mail can accomplish some good solutions. If your internal servers are running ldap then you can use that for your routing and filtering on the zentyal based mail gateway.
Its a learning curve thing or there is a support crew to make the time and effort a breeze.
Great thing about post fix is the wealth of infomation and when it comes to smpt there isn't much it cant do.
http://www.postfix.org/FILTER_README.html