In addition to authenticating your domain with Simplesat's email servers, you can add an SPF record to further improve email deliverability.
Simplesat uses SendGrid for email delivery that has an Automated Security feature. It automatically manages the DKIM and SPF records for you via the CNAME records that are added.
Please see their documentation from here.
If you have any questions about this or need any assistance please let us know. 👍