I'm seeing the same problem. Manifested itself in Mail.app reporting an ambiguous "problem" sending (Otherwise, it's a decent client, but I wish it'd give out "real" error messages).
Telnetting to port 25 and attempting to send a test message to myself manually resulted in the 450 error.
Apparently at this time, DH has been aware of the problem for ~4 hours, at least when I checked via support.
Sending via webmail seems to work, so there's a workaround. Annoying, but not critical.