The configuration of your email with FastMail depends on whether your domain is hosted with them or not. If your domain is hosted with FastMail then SPF and DKIM are automatically configured for you in DNS.
To configure SPF for your domain that is not hosted with FastMail you will have to include the below statement in your DNS.
v=spf1 include:spf.messagingengine.com ?all
If you already have an SPF record and need to add FastMail to it you will just have to include this part to your current record: include:spf.messagingengine.com
If your domain is hosted somewhere else you will need to make some entries in DNS in order to configure DKIM signing. You will need to create the following three CNAME records and replace the yourdomain with your domain.
Once the above is configured correctly, FastMail should be able to detect your records and show you the status.
For more information please click on the button below and see the instructions that relate to your setup with FastMail.
Create a free OnDMARC account to test your configuration.