Log in to Directadmin. If you are not sure how to get here, you can look in this guide.

Then navigate to DNS Manager -> MX Records

Press Add Record Here you fill in the data for the MX pointers that you get from your email provider.

1. Here it says MX default.
2. Here you write the name of your MX pointer
3. Enter the value for TTL
4. Here you enter priority
5. Finally, write the destination, where the MX pointer should point

Then click on Add.

Please note that the changes may take a while before they are implemented and working