smtpd-starttls-proxy is a chainloading program that runs right before the SMTP server on your command line. It interposes itself between the client commands and the server, and implements STARTTLS on behalf of the server.
Binary packages can be installed with the high-level tool pkgin (which can be installed with pkg_add) or pkg_add(1) (installed by default). The NetBSD packages collection is also designed to permit easy installation from source.
The pkg_admin audit command locates any installed package which has been mentioned in security advisories as having vulnerabilities.
Please note the vulnerabilities database might not be fully accurate, and not every bug is exploitable with every configuration.
Problem reports, updates or suggestions for this package should be reported with send-pr.