The configuration of your email with FastMail depends on whether your domain is hosted with them or not.

To set up SPF and DKIM for FastMail, Follow the steps given below:

SPF:

If your domain is not hosted with FastMail and if you want to configure then you will have to include the below statement in your DNS.

        v=spf1 include:spf.messagingengine.com

If you already have an SPF record and want to add Fastmail, then you will just have to include the above statement to your current SPF record: 

For Example:

If your previous SPF record was 

       v=spf1 mx -all 

your new SPF record will be

        v=spf1 mx include:spf.messagingengine.com -all  

DKIM:

• FastMail uses three CNAME records to support DKIM signing.
• You will need to create the following three CNAME records and replace the “mydomain.com” with your domain.