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msmtp is an SMTP client.

In the default mode, it transmits a mail to an SMTP server which takes care of further delivery.
To use this program with your mail user agent (MUA), create a configuration file with your mail account(s) and tell your MUA to call msmtp instead of /usr/sbin/sendmail.

Features include:

Sendmail compatible interface (command line options and exit codes)
Support for multiple accounts
TLS/SSL support including client certificates
Support for Internationalized Domain Names (IDN)
Many authentication methods
Fast SMTP implementation using command pipelining
DSN (Delivery Status Notification) support
SOCKS proxy support

msmtp runs on a wide variety of platforms. It needs little more than an ANSI C compiler and Berkeley-style sockets. It does not require any additional libraries, but it can use the following to enhance its feature set:

GnuTLS
The GnuTLS library provides TLS support. It is highly recommended. OpenSSL can be used as an alternative to GnuTLS, but this is discouraged.
GNU SASL
Using the GNU SASL library adds support for some exotic authentication methods. You really only need it if you want to use GSSAPI.
GNU libidn2
On modern Linux systems with recent glibc and GnuTLS libraries, msmtp has automatic support for Internationalized Domain Names (IDN). On other systems, IDN support is available via libidn2.
msmtp is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.