I was having issues sending emails from my android device using k9 mail. I have some pretty strong rules in /etc/postfix/header_checks file. To test if this was the cause I renamed the file. I then restarted and sent a test email.
I found that once I did that I received a different message when a test email failed to be sent. The error was “Error: queue file write error”.
Aug 8 10:24:19 mail postfix/cleanup[2324]: warning: regexp:/etc/postfix/header_checks is unavailable. open /etc/postfix/header_c
hecks: No such file or directory
So, you have one possible solution to that error. It is that header_checks is missing. When you perform tests like I did you should always ensure that you have a dummy file with that name. Restart the server and try another test email. Or find the reference to header_checks in one of the configuration files in postfix/dovecot and comment the line and restart.
Now I’m back to trying to figure out the cause of the original error message, and at least we have another documented cause of the queue file write error, which in my case had nothing to do with free space.