We recommend that you create and add a Sender Policy Framework (SPF) record for your domain. An SPF record is a type of Domain Name Service (DNS) record that identifies which mail servers are permitted to send email on behalf of your domain.
The purpose of an SPF record is to prevent spammers from sending messages with forged From addresses at your domain. Recipients can refer to the SPF record to determine whether a message purporting to be from your domain comes from an authorised mail server.
For example, suppose that your domain example.com uses Gmail. You create an SPF record that identifies the Google Apps mail servers as the authorised mail servers for your domain. When a recipient's mail server receives a message from email@example.com, it can check the SPF record for example.com to determine whether it is a valid message. If the message comes from a server other than the Google Apps mail servers listed in the SPF record, the recipient's mail server can reject it as spam.
If your domain does not have an SPF record, some recipient domains may reject messages from your users because they cannot validate that the messages come from an authorised mail server.
Refractiv will typically have recommended that you add an SPF record to your domain when your Google Apps account was set up, however it is worth checking yours is in place. To do so, either login to your DNS control panel or check your record by entering your domain in this MX Toolbox SPF Check.