  TLS is the most widely used cryptographic protocol on the Internet. While
many recent studies focused on its use in HTTPS, none so far analyzed TLS usage
in e-mail related protocols, which often carry highly sensitive information.
Since end-to-end encryption mechanisms like PGP are seldomly used, today
confidentiality in the e-mail ecosystem is mainly based on the encryption of
the transport layer. A well-positioned attacker may be able to intercept
plaintext passively and at global scale. In this paper we are the first to
present a scalable methodology to assess the state of security mechanisms in
the e-mail ecosystem using commodity hardware and open-source software. We draw
a comprehensive picture of the current state of every e-mail related TLS
configuration for the entire IPv4 range. We collected and scanned a massive
data-set of 20 million IP/port combinations of all related protocols (SMTP,
POP3, IMAP) and legacy ports. Over a time span of approx. three months we
conducted more than 10 billion TLS handshakes. Additionally, we show that
securing server-to-server communication using e.g. SMTP is inherently more
difficult than securing client-to-server communication. Lastly, we analyze the
volatility of TLS certificates and trust anchors in the e-mail ecosystem and
argue that while the overall trend points in the right direction, there are
still many steps needed towards secure e-mail.
