FastMail now enables opportunistic SSL/TLS encryption on incoming emails. This means we now advertise STARTTLS support in response to an EHLO command on our MX servers.
Trying 18.104.22.168...Connected to mx1.messagingengine.com.Escape character is '^]'.220 mx1.messagingengine.com ESMTP . No UCE permitted.EHLO blah.com250-mx1.messagingengine.com250-PIPELINING250-SIZE 71000000250-ETRN250-STARTTLS250-ENHANCEDSTATUSCODES250 8BITMIME
If the server sending the email to FastMail supports it, it will enable an SSL/TLS connection and send the email to FastMail over an encrypted connection. Some extra notes:
Received: from remoteserver.com (remoteserver.com [a.b.c.d]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.messagingengine.com (Postfix) with ESMTPS id 8FDBA27063B for <email@example.com>; Wed, 15 Apr 2009 23:28:29 -0400 (EDT)
Since this feature is entirely optional, it shouldn't affect any sending servers that don't support STARTTLS, and thus shouldn't affect the deliverability of email in any way.