A Solution to Postfix’s “unknown group: vmail” Error

Recently I migrated from one VPS to another, which involved re-installing many pieces of software and attempting to repair permissions on many files and directories. During the process of migrating, I started to notice errors appearing on my mail.err log file apparently thrown by postfix every couple of minutes, which looked like this:

postfix/pipe[4274]: fatal: get_service_attr: unknown group: vmail

For the solution, skip to the bottom.

The error at hand was preventing any emails from sending or receiving on my server. I had no idea what emails I was missing, and it was causing problems. Among other important emails, I started missing comment notifications on this website, and crash report notifications for my programs. I needed to find the solution, and I needed to find it fast.

Unsure of where I went wrong, I began to check the /etc/group file and the /etc/passwd files to make sure that both the vmail group and user existed — and they did. I tried everything I could think of to find the source of the error, from restarting the server, to reinstalling postfix and dovecot, to deleting the vmail user and group and re-adding them. I even attempted creating a whole new user and group, reassigning all the permissions, then changing all the vmail occurrences in every file I could find to the new group’s name. Nothing worked, and I was losing hope quickly.

With Google being no help, I turned to the Unix & Linux section of StackExchange for help. I posted a question and waited, but got no comments or answers after over 24 hours. My question quickly got buried and I decided that I couldn’t sit around and wait any longer.

I was really considering starting over and installing everything again (including Debian), but I decided to give it all one more look-over before I pulled the trigger and wiped everything. While staring blankly at the /etc/group file and wondering why the heck nothing was working, I suddenly realized that I had never checked the permissions of the file.

Solution:

I ran ls -l /etc/group and… OMG. The group file had no world read permissions. This whole time, postfix has been constantly trying to read the group file to check if the group existed, but was not allowed to because somewhere in the process of my VPS migration, I accidentally removed all world permissions from the group file. Running chmod 644 /etc/group and rebooting my server solved all my problems, and emails began to pile in from the nearly 5-day-old mail queue.