Normally, I can send email from commandline, such asecho hello | mail -s subject user@example.comI have noticed that even when Postfix is not running on my machine, I can still send emails. Actually they are being queued in /var/spool/postfix/maildrop. But once Postfix is started again, these emails will be sent out.Can somebody please explain how this works? Who copies the emails into /var/spool/postfix/maildrop, is it the email client (heirloom in my case) ?Does it mean, I have a guarantee all emails will be always delivered, regardless whether Postfix is running or not?
sending emails with heirloom, even when Postfix is not running locally
email;postfix;mail command
Can somebody please explain how this works? Who copies the emails into  /var/spool/postfix/maildrop, is it the email client (heirloom in my  case) ?Local mail submissions, are received with the Postfix sendmail compatibility command, are queued in maildrop queue by postdrop program.The postdrop command is designed to run with set-group ID privileges, so that itcan write to the maildrop queue directory and connect to Postfix daemon processes.Does it mean, I have a guarantee all emails will be always delivered,  regardless whether Postfix is running or not?Yes, postfix never drops mails even when its mail system is down. Other helper daemons like cron will check if the Postfix daemons are down for whatever reason, so postfix mails should not be lost, either. NoteHow Postfix receives mailpostdrop - Postfix mail posting utilitysendmail - Postfix to Sendmail compatibility interface