How to send an email from command line?
There are two programs that I am aware of which will easily allow you to configure your Mac to send email from the command line.

I have written up HOWTOs for both of them:


mailsend
msmtp


Of the two, I suggest msmtp.

Configuration is complicated enough that I'm not sure if I should replicate all of the steps here, but I will mention that if you use
Homebrew
you can install msmtp using

brew install msmtp --with-macosx-keyring

Then the rest is just a matter of setting up the related configuration files

The first is /usr/local/etc/msmtprc

# Begin msmtprc
# Set default values for all following accounts.
defaults
tls on
logfile ~/.msmtp.log

# A first gmail address
account example@gmail.com
host smtp.gmail.com
port 587
protocol smtp
auth on
from example@gmail.com
user example@gmail.com
tls on
tls_starttls on

# this next line is crucial: you have to point to the correct security certificate for GMail.
# If this doesn't work, check the mstmp documentation
# at http://msmtp.sourceforge.net/documentation.html for help
#
# This next line should all be on one long line:
tls_trust_file /path/to/Thawte Roots/Thawte SSLWeb Server Roots/thawte Premium Server CA/Thawte Premium Server CA.pem

# Set a default account
# You need to set a default account for Mail
account default : example@gmail.com

# end msmtprc


Note that tls_trust_file line should point to wherever you have downloaded and installed the certificates from https://www.thawte.com/roots/index.html.

I put mine in ~/Dropbox/Thawte Roots so that I can have it on all of my Macs.

Then you need a ~/.mailrc file to say where the msmtp binary is located. If you use brew it will be /usr/local/bin/msmtp so the file would look like this:

set sendmail="/usr/local/bin/msmtp"


The last but crucial step is making sure your Keychain has the information exactly in the format that msmtp will expect it:



I think that covers most of the details. See http://www.tuaw.com/2010/05/04/msmtp-a-free-tool-to-send-email-from-terminal/ if you want a few more specifics.