Problem: Mail rejected due to RBL.
Cause: Mail can be rejected by our servers if the IP address has been listed as a virus/trojan or spam source, this is for your protection.
Solution: Check on http://www.anti-abuse.org/multi-rbl-check/ incase the domain or IP address is listed for more detailed explaination.
Problem: Mail rejected due to 451 SPF lookup failure (#4.3.0).
Cause: When one mail server is talking to another mail server and it has SPF protection active then you need a matching SPF record on the server that is originating the email.
Solution: In cPanel you can active SPF in "Email Authentication" section, please include all IP addresses that would be sending mail on your behalf.