To allow Stackmail / 20i to send emails on your behalf, you will have to add them to your SPF record. The SPF mechanism used by Stackmail is shown below.
You can also add this record automatically through Account Preferences in My20i.
For more information please click the button below.
For instructions on how to configure DKIM please click on the button below.
Create a free OnDMARC account to test your configuration.