The stunnel program is designed to work as an SSL encryption wrapper between remote client and local (inetd-startable) or remote server. It can be used to add SSL functionality to commonly used inetd daemons like POP2, POP3, and IMAP servers without any changes in the programs‘ code. Stunnel uses the OpenSSL library for cryptography, so it supports whatever cryptographic algorithms are compiled into the library. 

Stunnel can benefit from FIPS 140-2 validation of the OpenSSL FIPS Object Module, as long as the building process meets its Security Policy. A scanned FIPS 140-2 Validation Certificate document is available for download on the NIST web page. The Windows binary installer is compiled with FIPS 140-2 support. The FIPS mode of operation is no longer enabled by default since stunnel 5.00. 

Stunnel is a free software authored by Michal Trojnara. Although distributed under GNU GPL version 2 or later with OpenSSL exception, stunnel is not a community project. We retain the copyright of the source code. Please contact us for commercial support or non-GPL licenses. Free, community-based support is also available via stunnel-users mailing list. 

The obsolete 3.x branch is no longer maintained. Use stunnel3 perl script as a drop-in replacement for backward compatibility.

reference from：https://www.stunnel.org/index.html