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Normally, I can send email from commandline, such as

echo hello | mail -s "subject" [email protected]

I 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?

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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 it can 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.

Note

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