If your domain and DNS are managed directly with us, your SPF records will be automatically created. If you're using an external DNS host and use our MX server, please be sure to set the spf record to:
v=spf1 a mx ip4:51.91.70.35 ip4:5.135.74.176 ip4:5.135.74.177 ip4:5.135.74.178 ip4:5.135.74.179 ip4:172.233.245.122 ip6:2600:3c07::2000:4fff:fea8:ba9e ~all