We provide email service with spam filtering as part of our hosting service, so it's usually not necessary to use an external email provider. However, if you must, it is possible, but we only support configuring it in one particular manner.
How DNS determines how email is handled
Within the dns zone for a particular domain name, one or more MX (mail exchanger) records specify which host(s) are responsible for handling email addressed to a user of the domain name.
An MX record must "point" to a hostname, not an IP address.
The right (and wrong) ways to set up external mail handling
The cPanel control panel system manages certain dns hostname records automatically for a hosted domain name, including the "mail" hostname -i.e. mail.yourhosteddomain.com (where yourhosteddomain.com is the domain name we host for you.) The cPanel system creates mail.yourhosteddomain.com as a hostname to use for both incoming and outgoing mail served by your MEDIASERVE hosting.
Many of our customers who wish to use an external mail service make the mistake of changing the mail hostname's IP address to point to their own mail server hosted on their organization's or third party's network, (via the dns zone editor in the control panel), and then set the MX record to point to the mail hostname (via the MX Entry area of the control panel.)
While this accomplishes redirecting email to be handled by the external server, it does so in a way that is not guaranteed to be preserved. Often we will move sites from server to server for load balancing purposes, or to upgrade hardware, which we like to do every 2-3 years.
When a site is moved from one server to another, the hostnames which cPanel automatically controls are recreated and pointed to IP addresses on the new server. Thus, you may change the mail hostname's IP address and operate successfully with that setting for a time, but suddenly find it is no longer working.
The proper way to handle this is to designate a hostname for the external server which will not get overwritten by cPanel. Since cpanel creates mail.yourhosteddomain.com to handle email internally, you may choose to use something like, smtp.yourhosteddomain.com, mail2.yourhosteddomain.com, extmail.yourhosteddomain.com, or anything else you choose. If there is already a hostname of a different domain name pointed to your mail server, you don't even have to create a new dns record - simply set that hostname in the MX Entry area of the control panel as the mail exchanger for your domain.
If, for example, you run your own mail server on your organization's network, and the server does not already have a valid internet hostname assigned, and the IP address which points to that mail server is 184.108.40.206:
The WRONG way to handle this would be to access the advanced dns zone editor in the control panel and change the mail record to point to the IP address 220.127.116.11, and then set the MX record in "MX Entry" to mail.yourhosteddomain.com.
The RIGHT way would be to create a brand new A (host) record, "smtp" for example, pointed to the IP address 18.104.22.168, then set the MX record in the MX Entry area of the control panel to smtp.yourhosteddomain.com.
Since cPanel doesn't manage smtp.yourhosteddomain.com directly, you won't have to worry about that record being changed during a server migration.
A related concern: Reverse DNS
If you edit your domain's zone file through cPanel's zone editor, and you point a hostname to the IP address of your mail server, it is also important that a reverse DNS record for the hostname is registered for the IP address. Most mail servers nowadays will not accept mail from a server without a Reverse DNS record matching the hostname it claims to be when delivering email messages. Typically the organization that is providing you with the IP address is responsible for Reverse DNS - so contact your ISP to set this up. Let them know you have designated smtp.yourhosteddomain.com (for example) as the hostname pointing to that IP, and you'd like a reverse DNS entry created.