One of the most commonly used email encryption extensions is STARTTLS. It is a TLS (SSL) layer over the plaintext communication, allowing email servers to upgrade their plaintext communication to encrypted communication. Assuming that the email servers on both the sender and the recipient side support encrypted communication, an eavesdropper snooping on the communication between the mail servers cannot use a sniffer to see the email contents. Similar STARTTLS extensions exist for the communication between an email client and the email server (see IMAP4 and POP3, as stated by RFC 2595). STARTTLS may be used regardless of whether the email's contents are encrypted using another protocol.