Added about 3 years ago
First thing, if you check this feature and get a message from a domain that doesn't have a SPF, mails will NOT be rejected. Said otherwise you wont lose mails from domains which don't have SPF.
Now SPF can be checked in
that way mails are rejected directly in SMTP stage (they don't go to quarantine) It does only apply to SPF fields ending with "-all"
SPF can also be checked in
In that case, mails failing SPF with "-all" are going to quarantine, mails failing SPF with other severities get a score associated to each case.
For now, you are in fact protected for SPF issues, this is done in SpamC.