Explanation for mail_queue_enter: create file maildrop/870989.992: Read-only file system

Sunday, May 10, 2026

Here is what happened after mail_queue_enter: create file maildrop/870989.992: Read-only file system.

NB I am still unable to send emails to the debian-user mailinglist. I sent an email to listmaster@lists.debian.org, but they too didn’t respond. That’s another story. EDIT: I thought that I was unable, but then it turned out that my message now passed. Although I had tried multiple times for several days in a row. And AFAIK I didn’t change anything in my DNS configs…

But the members of debian-users are really great: without waiting for further information from me, they started exploring my problem and discussed among themselves: https://lists.debian.org/debian-user/2026/05/threads.html

It’s Andy Smith who found the explanation, less than 24 hours after my post. His second post in the thread gave the correct explanation, which now helped me to find the solution.

The problem came because Systemd runs the monit service with ProtectSystem set to strict and without adding /var/spool/postfix/ to ReadWritePaths.

Here is what I had in my /usr/lib/systemd/system/monit.service:

ProtectSystem=strict
ReadWritePaths=/run/ /var/lib/monit/ /var/log/

I just added /var/spool/postfix/ to the list of read-write paths:

ReadWritePaths=/run/ /var/lib/monit/ /var/log/ /var/spool/postfix/

How did I know that it’s /var/spool/postfix where maildrop tries to write to? Because the error message unfortunately specifies only the relative path. I guessed that this path is specified somewhere in the postfix configs, so I said:

# postconf | grep queue

Which returned a few settings, and one of them was:

queue_directory = /var/spool/postfix

My congratulations to the debian-users list. If only I could tell them all this!