sprint.net Email problems?

Hi, depending on the IP address space from where I'm trying to reach the two MX for @sprint.net, I'm getting either: - no TCP connection at all (Connection refused) - a TCP session, but not even a SMTP greeting banner - a SMTP session, but as response to RCPT TO a 550 Access denied For the third case, doesn't seem to matter which email address I try (did ipv6-support@, rrockell@ and postmaster@). This is persistant now for over 12 hours... Best regards, Daniel