> [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. Actually, I consider this approach as unacceptable unless the second delivery attempt occurs within a minute or two. (Which, incidentally, is a much shorter retry period after deferral than the standards recommend.) Ned