What is the current state of the law on "knowingly" mailing a communication that contains any threat to kidnap or injure the addressee?

To reiterate, 876(c) criminalizes "knowingly" mailing a communication that "contain[s] any threat to kidnap any person or any threat to injure the person of the addressee or of another." 18 U.S.C. 876(c). Other provisions in 876 criminalize actions "with intent to extort," but subsection (c) contains no such language. Compare id. 876(b) & (d), with id. 876(c). Thus, at first glance, 876(c) may seem to punish any individual who knowingly sends a threat in the mail even if he or she had no intent to threaten. But case law has fleshed out this element and merits clarification of our court's precedent surrounding 876(c)'s mens rea requirement. Initially, we added to 876(c)'s statutory elements and "inferred . . . a general intent to threaten [a]s an essential element of the crime." United States v. LeVision, 418 F.2d 624, 626 (9th Cir. 1969). Several years later, we described 876(c)'s requirements without reference to an intent to threaten. United States v. Sirhan, 504 F.2d 818, 819 (9th Cir. 1974) (per curiam) ("First, the defendant must have written and mailed a letter (or other communication) containing a threat to injure another person. Secondly, he must have knowingly caused the letter to be deposited in the mails."). After that, we reaffirmed an intent-to-threaten element in 876(c) as "a showing of specific intent" to threaten. United States v. Twine, 853 F.2d 676, 679-80 (9th Cir. 1988). And shortly thereafter, we explained that "[t]he only proof of specific intent required to support a conviction under 18 U.S.C. 876 is that the defendant knowingly deposits a threatening letter in the mails, not that he intended or was able to carry out the threat." United States v. Davis, 876 F.2d 71, 73 (9th Cir. 1989) (per curiam) (emphasis added) (citation omitted).