Stunnel is used to provide secure encrypted connections for clients or servers that do not speak TLS or SSL natively.

Stunnel relies on the OpenSSL library to implement the underlying TLS or SSL protocol.

[citation needed] Stunnel is maintained by Polish programmer Michał Trojnara and released under the terms of the GNU General Public License (GPL) with OpenSSL exception.

[7] A stunnel can be used to provide a secure SSL connection to an existing non-SSL-aware SMTP mail server.

[citation needed] The stunnel process could be running on the same or a different server from the unsecured mail application; however, both machines would typically be behind a firewall on a secure internal network (so that an intruder could not make its own unsecured connection directly to port 25).