Below mentioned are the steps which needs to be performed whenever you want to set your own custom MX records for your domain:
1) Login into HELM Control Panel.
2) Click on Domains button.
3) Click on Hosted Domain for which one need to set custom MX record.
4) Click on DNS Zone Editor.
5) Click on the existing Mail A record and Delete.
6) Click on the existing MX record and Delete.
7) Click Add New button. Choose record type as A. For the record name, enter the hostname of mail server and save it.
8) Click Add New button. Choose record type as MX. Enter @ for the record name. For the record data, enter the fully qualified domain name (for e.g mail.domain.com) of mail server and save it.