550 5.7.23 SPF validation failed is an error message indicating that the sender's domain did not pass the sender policy framework (SPF) validation check. This is a security measure to prevent email spoofing, which is when a malicious sender uses a fake email address to make it appear as though the email is coming from a legitimate sender. The "Reason: mechanism" part of the error message indicates that the SPF validation failed because the mechanism specified in the sender's SPF record is not valid or is not supported by the recipient's email server. To fix this issue, the sender will need to update their SPF record to use a valid mechanism and ensure that it is formatted correctly.