The most important thing you need for email forwarding to work is MX records for your domain. Without MX records, your domain cannot receive emails.
The MX record usually requires 2 records. One A record: mail.yourdomain.com for example which points to your IP address and the MX record where you assign your newly created subdomain. By default, the "priority" field for your MX record is 0.
When you are using private nameservers then out system has already created MX record for you. If you are using any other nameserver provider (external DNS host) then set up the MX record with them. If you are not sure how to set up a MX record then contact your DNS provider who can help you set it up on their platform.