How to send email from Terminal?

Probably the simplest way is to use curl for this, there is no need to install any additional packages and it can be configured directly in a request.

Here is an example using gmail smtp server:

curl --url 'smtps://smtp.gmail.com:465' --ssl-reqd \
  --mail-from 'from-email@gmail.com' \
  --mail-rcpt 'to-email@gmail.com' \
  --user 'from-email@gmail.com:YourPassword' \
  -T <(echo -e 'From: from-email@gmail.com\nTo: to-email@gmail.com\nSubject: Curl Test\n\nHello')

If all you need is a subject line (as in an alert message) simply do:

mailx -s "This is all she wrote" < /dev/null "myself@myaddress"

If you want to attach a file on Linux

echo 'mail content' | mailx -s 'email subject' -a attachment.txt username@stackoverflow.com

in the terminal on your mac os or linux os type this code

mail -s (subject) (receiversEmailAddress)  <<< "how are you?"

for an example try this

mail -s "hi" abc@example.com <<< "how are you?"<br>

For SMTP hosts and Gmail I like to use Swaks -> https://easyengine.io/tutorials/mail/swaks-smtp-test-tool/

On a Mac:

brew install swaks swaks --to user@example.com --server smtp.example.com

I think swaks is the best. Here you have more complicated example, using TLS encryption on port 25:

swaks --from john.smith@mydomain.com \
--h-From: '"John Smith" <john.smith@mydomain.com>' \
--h-Subject: 'Subject of message' \
--auth LOGIN --auth-user mylogin --auth-pass mypass \
--to someone@otherdomain.com \
--server smtp.example.com --port 25 -tls \
--add-header 'Content-Type: text/plain; charset="utf-8"'