SPF Validation Error

Status code: 550 5.7.23
received-spf: None (protection.outlook.com: senderdomain.co.uk does not designate permitted sender hosts)

Cause
Incorrect TXT SPF Record.

Solution:

Check the entry thoroughly. It should look like this:

v=spf1 include:spf.protection.outlook.com -all

Sometimes it may be missing a “-” before “all”. This has to be present.