msmtp

msmtp是一个开源的SMTP客户端，在默认模式下，帮助您传输邮件的SMTP服务器。

In the default mode, it transmits a mail to an SMTP server (for example at a free mail provider) 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.