How to set field permission programmatically in Drupal?

Want to set an e-mail filter on the X-Apparently-To: field?

• I have filters and an alias set up because my newly-created UID immediately began receiving large quantities of junk mail. Now spammers are using the X-Apparently-To: field to circumvent my To: filters, and the filter tool doesn't let me check this field. Ideally it ought to let me choose any field in the entire header, but at a minimum it should include any field that can be used to identify a recipient or otherwise direct delivery into a mailbox. Otherwise the account loses much of its usefulness. How did a new (never existed before) UID come under such heavy abuse in the first place?

• Answer:

  You should be able to type in the field in the filter box, but whether it will work or not, I don't know. I'm not sure that the filter checks this section. However, you could just keep marking them as spam and hope that it works.