On Monday night, the server holding the disk array for our /home/users had a crash. Our email server (debian, running postfix) is on another machine which NFS mounts the user disk - meaning the mail server kept running.

This means that email was coming in, being processed - then when postfix tried to write the message to ~/Maildir/new - it failed.

Q1: the mail.log files don't tell me what kind of error code got sent back to the sender of the message. I don't know if they got a 400, 450, 500, etc. I DO know that the senders received a "bounce" message - and that their servers did not attempt to resend the message. This seems to indicate to me that postfix sent a 500-level error message - right?
The mail.log literally shows: status=bounced (can't create user output file)

Q2 (minor issue): is there a setting in the postfix configuration that will put the actual error code sent back to the sender into the log file?

Q3 (the real reason for my post): What is the appropriate error code that postfix SHOULD return to the sender? Iow - should the senders have gotten a 400 level code so that the message could have been retried?