On Thu, Feb 04, 2016 at 07:00:44PM -0800, Ned Freed wrote:
Yes, of course with cleartext transmission in the absence of STARTTLS
support. I had expected that would have been clear from context.
That's in no way sufficient. Not only do you have to be willing to do without
STARTTLS, you also have to be willing to close the connection and try another
in the event that the server offers STARTTLS, the client attempts to use it
but
the TLS negotiation fails for some reason.
This is true, reasonably well known[1] and largely tangential to
the topic of this thread which is ietf.org disabling SSLv3 support
in SMTP STARTTLS. My point is that this action, be it mostly
symbolic, is at this time harmless as the overwhelming majority of
TLS-capable servers and clients can now do TLSv1 or better.
--
Viktor.
[1] http://www.postfix.org/TLS_README.html#client_tls_may
With opportunistic TLS, mail delivery continues even if
the server certificate is untrusted or bears the wrong
name. When the TLS handshake fails for an opportunistic
TLS session, rather than give up on mail delivery, the
Postfix SMTP client retries the transaction with TLS
disabled. Trying an unencrypted connection makes it possible
to deliver mail to sites with non-interoperable server TLS
implementations.
The implementation and documentation of this was joint work with
Wietse back in early 2006. These days, when STARTTLS fails, Postfix
tries other MX hosts first and if they all fail, defers the mail
initially. Cleartext fallback kicks in on the second delivery
attempt if STARTTLS fails again.