Modify Subject is an additional option that should be used to help the end users be aware of the elevated risk of interacting with these messages. Modify Subject allows the administrator to add a prefix or suffix to the message subject, such as "[SPF Fail]", to indicate that the message has failed the SPF verification and may be fraudulent or spoofed.
The other options are not valid additional options to help the end users be aware of the elevated risk of interacting with these messages, because they do not affect the message subject or provide any warning to the end users.