One-stop resource on how to effectively disable SSLv3 in major web browsers as well as in web, mail and other servers that may still be using it.
Put the following lines in your sendmail.mc configuration file, in the LOCAL_CONFIG section:
LOCAL_CONFIG O CipherList=HIGH O ServerSSLOptions=+SSL_OP_NO_SSLv2 +SSL_OP_NO_SSLv3 +SSL_OP_CIPHER_SERVER_PREFERENCE O ClientSSLOptions=+SSL_OP_NO_SSLv2 +SSL_OP_NO_SSLv3
Then restart the server (in Ubuntu: sudo service sendmail restart).