Using Jessie but need a generic Debian solution

Requirement is simple.

I need a way to send one-line email messages from the command line.

I have set up a gmail account just for this particular Rpi3, with the address of [email protected] - Claws email client works just fine with it, using TLS and no 2FA.

So now I need to be able to send one-line mail messages from anywhere without user intervention.