To authorise iomart to send emails on your behalf of your domain, add their mechanism to your SPF record.
The mechanism for iomart email is:
iomart supports DKIM authentication. To enable DKIM for your custom domain please contact iomart here: https://www.iomart.com/contact
Testing Your Configuration
To test your configuration for both SPF and DKIM is correctly configured, start your 14 day free trial with OnDMARC!