SPF (Sender Policy Framework) Records are a type of DNS record which is used to assist in the prevention of unauthorised and potentially spoofed email sending. This type of DNS record in particular (which runs under the TXT Record type) defines which mail server or servers should be permitted to send email on behalf of a domain name either using specific host(s) or IP addresses.
Modern day email applications check the server which sent the email against any these; if the application detects that the sender does not match the SPF Record, it will often reject the email with the following reason which gets returned to the sender as a bounceback;
550 SPF: x.x.x.x is not allowed to send mail from domain.com
If you are trying to send emails and are encountering this sort of bounceback message, you may like to try checking your DNS Records to see if an SPF has been defined. This can be done using many sites, such as MXToolbox.