As part of the Sentinel Messaging spam scanning, the senders SPF record can contribute to the overall spam score of the message. Sentinel Messaging does not block an email outright for SPF entry. This is because there are a large number of domains that have an incorrect SPF record.
If you see SPF failure mentioned in the Delivery Status it is likely due to the destination mail server checking the SPF record. Please review your mail server for any type of SPF check scanning.