I have configured the Zend Mailman plugin to use SMTP host 127.0.0.1 port 25, (Postfix as local mailer) and set "Type of Encription" (typo!) to "None". Also no SMTP authentication is set.
The local mail.log shows following:
postfix/smtpd[25561]: connect from localhost[127.0.0.1]
postfix/smtpd[25561]: SSL_accept error from localhost[127.0.0.1]: 0
postfix/smtpd[25561]: warning: TLS library problem: error:14094418:SSL routines:SSL3_READ_BYTES:tlsv1 alert unknown ca:s3_pkt.c:1262:SSL alert number 48:
postfix/smtpd[25561]: lost connection after STARTTLS from localhost[127.0.0.1]
postfix/smtpd[25561]: disconnect from localhost[127.0.0.1]
Seems like the "None" setting does not disable SSL/STARTLS.