I’ve occasionally found it useful to set up reminders to be emailed to myself on a regular basis. The most straightforward tool to use for this is cron with mail. More here
If you’re sending directly from the command line, this command:
mail -s “Subject test” firstname.lastname@example.org
will not actually send the mail straight away. Instead, it will wait for you to type in the body of the email (which can be empty), and hit Ctrl-D twice.