Some numbers on our inbound and outbound TLS usage in SMTP
As a result of POODLE,
it's suddenly rather interesting to find out the volume of SSLv3
usage that you're seeing. Fortunately for us, Exim directly logs
the SSL/TLS protocol version in a relatively easy to search for
format; it's recorded as the '
X=...' parameter for both inbound
and outbound email. So here's some statistics, first from our external
MX gateway for inbound messages and then from our other servers for
Over the past 90 days, we've received roughly 1.17 million external email messages. 389,000 of them were received with some version of SSL/TLS. Unfortunately our external mail gateway currently only supports up to TLS 1.0, so the only split I can report is that only 130 of these messages were received using SSLv3 instead of TLS 1.0. 130 messages is low enough for me to examine the sources by hand; the only particularly interesting and eyebrow-raising ones were a couple of servers at a US university and a .nl ISP.
(I'm a little bit surprised that our Exim doesn't support higher TLS versions, to be honest. We're using Exim on Ubuntu 12.04, which I would have thought would support something more than just TLS 1.0.)
On our user mail submission machine, we've delivered to 167,000 remote addresses over the past 90 days. Almost all of them, 158,000, were done with SSL/TLS. Only three of them used SSLv3 and they were all to the same destination; everything else was TLS 1.0.
(It turns out that very few of our user submitted messages were received with TLS, only 0.9%. This rather surprises me but maybe many IMAP programs default to not using TLS even if the submission server offers it. All of these small number of submissions used TLS 1.0, as I'd hope.)
Given that our Exim version only supports TLS 1.0, these numbers are more boring than I was hoping they'd be when I started writing this entry. That's how it goes sometimes; the research process can be disappointing as well as educating.
(I did verify that our SMTP servers really only do support up to TLS 1.0 and it's not just that no one asked for a higher version than that.)
One set of numbers I'd like to get for our inbound email is how TLS usage correlates with spam score. Unfortunately our inbound mail setup makes it basically impossible to correlate the bits together, as spam scoring is done well after TLS information is readily available.
Sidebar: these numbers don't quite mean what you might think
I've talked about inbound message deliveries and outbound destination
addresses here because that's what Exim logs information about, but
of course what is really encrypted is connections. One (encrypted)
connection may deliver multiple inbound messages and certainly may
be handed multiple
RCPT TO addresses in the same conversation.
I've also made no attempt to aggregate this by source or destination,
so very popular sources or destinations (like, say, Gmail) will
influence these numbers quite a lot.
All of this means that this sort of numbers can't be taken as an indication of how many sources or destinations do TLS with us. All I can talk about is message flows.
(I can't even talk about how many outgoing messages are completely protected by TLS, because to do that I'd have to work out how many messages had no non-TLS deliveries. This is probably possible with Exim logs, but it's more work than I'm interested in doing right now. Clearly what I need is some sort of easy to use Exim log aggregator that will group all log messages for a given email message together and then let me do relatively sophisticated queries on the result.)