> Is it against the law to send someone a letter in the U.S. mail with the F-word, the S-word, the C-word etc.
>
> There is literally nothing on the internet that says whether it is or isn't.
I doubt very much that there will be any problem about such letters
if the recipient doesn't object to receiving it and it doesn't end
up posted on the Internet. I'd expect husbands and wives separated
(by, say, overseas deployment or long business trips) might communicate
about what they want to do to each other when they get back together
and it might sound downright pornographic. Assuming that "the C-word"
is "condom", the Catholic Church would probably object to that word,
but why would they ever see such a letter between husband and wife?
On the other hand, if you are stalking someone, that someone could
take your letter to the police and accuse you of threatening rape,
some other violent attack, or murder. Even if it doesn't result
in a conviction, you could end up with a ruined reputation, a dim
political future, or loss of job (especially if you are a teacher
or caregiver for children). And you might lose custody of your
children.
Note that the US Postal Service has long been used to deliver
magazines in plain brown wrappers that not only use "swear" words,
but illustrate them with photos using zoom lenses, especially before
the invention of Internet Porn sites.
There will always be some idiot who claims that using the F-word
or S-word or C-word is taking the name of the Lord in vain (whose
Lord? Mine certainly doesn't have a name like that!)