Function mail_internals::grammar::is_vchar

pub fn is_vchar(ch: char, mt: MailType) -> bool

VCHAR as defined by RFC 5243

Is true if it's either an us-ascii vchar or an non us-ascii char and the mail type is internationalized.

This mean that this includes non printable characters as long as the mail is internationalized and the character is non us-ascii utf-8.