I have a Ubuntu 16.04 LTS & Win 10 dual boot desktop PC .
I want to be able to send a mail to 1 or many accounts (mainly gmail.com) from this Linux using the console .
The command has to be intuitive and sort of a "template", something like
mail receivers -s "this is the subject " " this is the message" ...
Something simple. How can I achieve that?
"Simple" is a relative term. Simple to me might be very difficult to someone else.
The simplest way to send (and receive) e-mail is by using Evolution, Thunderbird or one of the other e-mail clients available.
You can send e-mail from the command line and, in fact, many programs (e.g. monitoring software) e-mail information in just this way. However, setting this up is non-trivial.
Based on the wording of your question it sounds like this is what you want to do. If so, you have some reading and some work ahead of you.
mailx seems to be installed in one of my 16.04 machines. It is not installed in the other. So you may or may not have to install it. More information about how to use mailx can be found here https://www.computerhope.com/unix/umailx.htm
There are other CLI mail clients such as mail, s-nail and so on. I happen to like mailx. Each has it's own features so in your research in testing you will probably find one that you like better than the others.
You will also need to get a SMTP server going. Postfix is the most popular but sendmail is a lot easier to set up. There are plenty of guides out there on how to do this.
Also note that you'll have problems just setting up a random machine and sending mail out from it. If you are not a legitimate Mail Transfer Agent (MTA) you will find your e-mail getting bounced a LOT as spam.
Certainly a worthy project as an educational exercise though!
