>The justification for MUST was I suspect to prevent infinite recursion mail >loops. For that particular purpose a count is sufficient. Oh, of course, you're right, it even says so in section 6.2 of RFC 2821, now 6.3 in RFC 5321. But there isn't a count, so this would be an incompatible change to the SMTP protocol, which seems like a non-starter to me. R's, John