Recent attacks against the DNS infrastructure have exposed the limitations of relying on TLS/SSL certificates for securing connections on the Internet.

Given that typical mail servers don't rotate their keys very often, it's not too cumbersome to hardcode their fingerprints and prevent your mail software from connecting to them should the certificate change. This is similar to how most people use ssh: assume that the certificate is valid on the first connection, but be careful if the certificate changes afterwards.

Fetchmail

Here's how to specify a certificate for a POP/IMAP server (Gmail in this example).

First of all, you need to download the server certificate:

openssl s_client -connect pop.gmail.com:995 -showcerts  
openssl s_client -connect imap.gmail.com:993 -showcerts

Then copy the output of that command to a file, say gmail.out, and extract its md5 fingerprint:

openssl x509 -fingerprint -md5 -noout -in gmail.out

Once you have the fingerprint, add it to your ~/.fetchmailrc:

poll pop.gmail.com protocol pop3 user "remoteusername" is "localusername" password "mypassword" fetchall ssl sslproto ssl3 sslfingerprint "12:34:AB:CD:56:78:EF:12:34:AB:CD:56:78:EF:12:34"

Postfix

Similarly, to detect changes to the certificate on your outgoing mail server (used as a smarthost on your local postfix instance), extract its sha1 fingerprint:

openssl s_client -connect mail.yourisp.net:465 -showcerts  
openssl x509 -fingerprint -sha1 -noout -in isp.out

Then add the fingerprint to /etc/postfix/main.cf:

relayhost = mail.isp.net  
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd  
smtp_sasl_auth_enable = yes  
smtp_sasl_security_options = noanonymous  
smtp_tls_security_level = fingerprint  
smtp_tls_mandatory_ciphers = high  
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3  
smtp_tls_fingerprint_digest = sha1  
smtp_tls_fingerprint_cert_match =  
   12:34:AB:CD:56:78:EF:90:12:AB:CD:34:56:EF:78:90:AB:CD:12:34