IN THE Federalist days, the young, fretful American Republic tried to stop pamphlets about the French Revolution from reaching the mailboxes of its citizens. Since then, in sporadic moments of crisis, the federal government has continued to screen mail and weed out what it considers foreign or dangerous propaganda. Such moments have come in the pre-Civil War days, during the two World Wars, and, now, in the Cold War. But the present little-known program, a joint effort by the Post Office Department and the Customs Bureau, is facing the heaviest attack in its existence. For the first time, law suits have been filed against it. Under the program, the Customs Bureau checks foreign non-first class mail as it enters the United States. If translators and inspectors decide the mail contains foreign - usually Communist - political propaganda, the Post Office generally holds it up and sends a notice to the addressee...