If you are using an external DNS provider then the correct way to set up an external mailserver is firstly to create an A record with a hostname pointing at the nameservers IP address, for example:
mail.yourdomain.com. A 123.123.123.123
Once this is done you should then set the MX record to point at this A record, as follows:
0 MX mail.yourdomain.com.
You should not set the MX record to an IP address directly as this is incorrect, an MX record is meant to point to a hostname and can behave incorrectly when pointed to an IP address. If you are using NetHosted nameservers and you wish to make this change simply submit a ticket explaining this with the external mailserver IP and we will make this change for you. This applies to shared hosting accounts, if you have a reseller hosting account you can make this change yourself via your WHM hosting account.
Please note that when using an external mail server you'll need to ensure the Email Routing setting is set to "Remote Mail Exchanger" in cPanel's Email Routing section.
