MMC, it is possible to tailor the networking to require, request or permit traffic to various domains and protocols to use encryption. E-mail Opportunistic Encryption can also be used for specific traffic like e-mail using the SMTP STARTTLS extension for relaying messages across the Internet, or the Internet Message Access Protocol (IMAP) STARTTLS extension for reading e-mail. With this implementation, it is not necessary to obtain a certificate from a certificate authority, as a self-signed certificate can be used. Using TLS with IMAP, POP3 and ACAP SMTP Service Extension for Secure SMTP over TLS STARTTLS and postfix STARTTLS and Exchange Many systems employ a variant with third-party add-ons to traditional email packages by first attempting to obtain an encryption key and if unsuccessful, then sending the email in the clear. PGP, p≡p, Hushmail, and Ciphire, among others can all be set up to work in this mode. In practice, STARTTLS in SMTP is often deployed with self-signed certificates, which represents a minimal one-time task for a system administrator, and results in most email traffic being opportunistically encrypted. VoIP Some Voice over IP (VoIP) solutions provide for painless encryption of voice traffic when possible. Some versions of the Sipura Technology and Linksys lines of analog telephony adapters (ATA) include a hardware implementation of SRTP with the installation of a certificate from Voxilla, a VoIP information site. When the call is placed an attempt is made to use SRTP, if successful a series of tones are played into the handset, if not the call proceeds without using encryption. Skype and Amicima use only secure connections and Gizmo5 attempts a secure connection between its clients. Phil Zimmermann, Alan Johnston, and Jon Callas have proposed a new VoIP encryption protocol called ZRTP. They have an implementation of it called Zfone whose source and compiled binaries are available. Websites For encrypting WWW/HTTP connections, HTTPS is typically used, which requires strict encryption and has significant administrative costs, both in terms of initial setup and continued maintenance costs for the website operator. Most browsers verify the webserver's identity to make sure that an SSL certificate is signed by a trusted certificate authority and has not expired, usually requiring the website operator to manually change the certificate every one or two years. The easiest way to enable some sort of opportunistic website encryption is by using self-signed certificates, but this causes browsers to display a warning each time