As of Postfix 2.11 the preferred mechanism for session resumption
is RFC 5077 TLS session tickets, which don't require server-side
storage. Consequently, for Postfix >= 2.11 this parameter should
generally be left empty. TLS session tickets require an OpenSSL
library (at least version 0.9.8h) that provides full support for
this TLS extension. See also smtpd_tls_session_cache_timeout.