Yes and no
.
You cannot have two servers answering for one fully qualified domain name.
Presuming the website is at
www.example.com and they want to get email at
[email protected], their system at their office cannot be responding for example.com and your DA server cannot be responding for mail.example.com.
So let's presume their server responds as mail.example.com...
Who is authoritative for DNS for their domain? Do they, or their ISP? If you're not authoritative for DNS for their domain, then you should probably not have DNS records on your DA server for the domain at all.
Whover is authoritative for DNS for their domain should have an A record for example.com pointing to your server, an A record for
www.example.com pointing to your server or a CNAME record for
www.example.com pointing to example.com, and an MX record for example.com pointing to mail.example.com (it appears that you can do all this through the DA control panel should you be authoritative for DNS for the domain).
The one last step, which must be taken whether or not you're authoritative for DNS for the domain, is that you need to make sure the domain name is NOT in the file /etc/virtual/domains. This change must, if required, be made by the server administrator, using root shell access.
If this last step isn't taken, then mail created on the server (for example, administration messages, or mail from someone using the server for outgoing smtp, or mail from forms on the domain website) will not make it to the real domain mailserver.
Jeff