What Postfix TLS support does for youTransport Layer Security (TLS, formerly called SSL) provides certificate-based authentication and encrypted sessions. An encrypted session protects the information that is transmitted with SMTP mail or with SASL authentication.NOTE: By turning on TLS support in Postfix, you not only get the ability to encrypt mail and to authenticate remote SMTP clients or servers. You also turn on hundreds of thousands of lines of OpenSSL library code. Assuming that OpenSSL is written as carefully as Wietse’s own code, every 1000 lines introduce one additional bug into Postfix.Topics covered in this document:How Postfix TLS support worksSMTP Server specific settingsSMTP Client specific settingsTLS manager specific settingsBuilding Postfix with TLS supportReporting problemsCreditsAnd last but not least, for the impatient:Getting started, quick and dirty

Source: Postfix TLS Support