There are several different accepted methods in use now that keep spammers from being successful. One of those methods is the SPF designation. In order for your emails not to be rejected by the recipients mail system we recommend that you create 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 authorized 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 authorized mail servers for your domain. When a recipient's mail server receives a message from firstname.lastname@example.org, 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 Gmail's mail servers listed in your domain's 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 authorized mail server.
To Create an SPF record for your domain, you will have to be able to edit your domains DNS information and use a format that specifies your mail servers.