NBSMTP

From nbSMTP manpage:

nbSMTP is a lightweight SMTP client. It simply takes a message from STDIN and sends it to a relayhost. A relayhost is meant to be a full SMTP server and it will really send the message.

Installation

Install nbsmtp^AUR package.

Forward to a Gmail Mail Server

To configure nbSMTP, you will have to edit its configuration file (~/.nbsmtprc) and enter your account settings:

~/.nbsmtprc

relayhost=smtp.gmail.com
port=587
use_starttls=True
fromaddr=myusername@gmail.com
auth_user=myusername@gmail.com
auth_pass=myultrasecretpassword

Be careful with permissions on this file, it is recommendable to run this:

chmod 600 ~/.nbsmtprc

To test the configuration, create a file (testemail):

testemail

To: myusername@gmail.com
From: myusername@gmail.com
Subject: nbsmtp test
hello email world

and then run:

/usr/bin/nbsmtp < testemail