Problem Description
While using Google Apps, sending email results in a bounce about SPF and that an IP is not allowed to send mail from the domain
Problem Resolution
This occurs because of Sender Policy Framework (SPF). SPF defines what servers are allowed to send email on behalf of your domain name and, by default, only allows the server that your website resides upon to send email using your domain. To resolve this, make sure that your SPF DNS record has include:_spf.google.com in it.
There are other records that could cause issues like this and so you should ensure every step in our guide to configuring Plesk for external mail services has been completed.