Thanks, Till. That makes sense.
However, when I try to add those directives to master.cf, Postfix complains about a syntax error:
fatal: /etc/postfix/master.cf: line 136: bad transport type: =
in response to this line:
smtpd_tls_security_level = encrypt
It seems that master.cf requires a different syntax than main.cf.
Any thoughts on this?