For many years there was ‘blat’, the all-purpose command-line mailer for Windows. However, blat was never updated for SSL, and a great many email providers (e.g.,. Gmail) require SSL. One can add a second tool called ‘stunnel’ to create a chain for the purpose, but I don’t like to kludge that much.
For a while I preferred the applet ‘email’, which is part of Cygwin. However, this is not a standalone applet, and there have been other issues. Just now SwithMail came to light for me:
http://www.tbare.com/software/swithmail.html
Am testing. So far, it is working very well indeed.