If the SMTP fixup is disabled, what happens?

A.
All SMTP commands are allowed to mail servers, and they are no longer protected from known
security problems with some mail server implementations.

B.
Only the SMTP commands specified in RFC 821 section 4.5.1 are allowed to a mail server:
HELO, MAIL, RCPT, DATA, RSET, NOOP, and QUIT.

C.
A safe conduit exists for SMTP connections from the outside to an inside e-mail server.

D.
All SMTP commands are allowed to a mail server, but mail servers are still protected from
known security problems with some mail server implementations.