I have a VPS with ubuntu.
1.I don't need postfix so I shut it down. Today I saw it is up again. how can I stop it completely? does a sudo service postfix stop enough?
What about dovecot? Do I need it or can I stop it as well?
2.I have 40+ users in usermin. I only use root but maybe other processes use other users? How can I know which user I can remove? Or if it's not me who created it, I can delete it?
3.auth.log and mail.log were too big so I deleted them and they were not recreated. I created auth.log and sudo chown syslog:adm /var/log/auth.log but it is still empty. What should I do so those files will be created with logs?
Thanks
You should be really making a separate post for each of your questions, otherwise this will be to broad. You also should provide more detail regarding the VPS; did you install it yourself? Which version of Ubuntu are you running?
As an answer to 1) Since 16.04 the canonical (haha) way of disabling services is typically using systemd, so
systemctl disable <service-name>...Ad 2) if it's your server, and you're looking after it, you should know what users you have and whether they need to be there. Are you talking about system accounts (users with IDs smaller than 1000) that are running certain processes? Are they regular user accounts?
Ad 3) Try restarting (r)syslog.