Sender Policy Framework (SPF), "is an email validation system designed to prevent email spam by detecting email spoofing, a common vulnerability, by verifying sender IP addresses.” (Wikipedia) Basically, SPF tells the Internet which IP addresses your domain name permits email to be sent from. If an email receiver receives a message purportedly from your domain, and the sending IP address is not permitted via your SPF record, then the receiver can reject that message.
Fundamentally, then, if you wish to have email from your domain sent through a third party service, you must configure your SPF record to permit delivery from the service provider’s IP addresses. If you don’t do this, then you risk having email receivers reject all email sent from your domain.
Fortunately, setting up the MailChannels SPF record is easy. All you have to do is add “include:relay.mailchannels.net” to your SPF record, and you’re done. If you have thousands of domains under management, changing the SPF records can be a touch more challenging, but unfortunately there is no way around this requirement in order to use our outbound spam filtering service.