Send a message with the content of message.txt to the mail directory of local user username

Send email from the command-line.
More information: <https://manned.org/sendmail>.
Trick copyright: tl;dr; <https://github.com/tldr-pages/tldr>

sendmail {{username}} < {{message.txt}}

click the source code to copy install sendmail on any operating system with command-not-found.com