The Electronic Frontier Foundation reports on disturbing research concerning Internet Service Providers and the privacy of their customers’ e-mail:
Another network-tampering threat to user safety has come to light from other providers: email encryption downgrade attacks. In recent months, researchers have reported ISPs in the US and Thailand intercepting their customers’ data to strip a security flag—called STARTTLS—from email traffic. The STARTTLS flag is an essential security and privacy protection used by an email server to request encryption when talking to another server or client.
By stripping out this flag, these ISPs prevent the email servers from successfully encrypting their conversation, and by default the servers will proceed to send email unencrypted. Some firewalls, including Cisco’s PIX/ASA firewall do this in order to monitor for spam originating from within their network and prevent it from being sent. Unfortunately, this causes collateral damage: the sending server will proceed to transmit plaintext email over the public Internet, where it is subject to eavesdropping and interception. [...]
It is important that ISPs immediately stop this unauthorized removal of their customers’ security measures. ISPs act as trusted gateways to the global Internet and it is a violation of that trust to intercept or modify client traffic, regardless of what protocol their customers are using. It is a double violation when such modification disables security measures their customers use to protect themselves.