cannot chdir to domain dir domain.com: Permission denied (Plesk postfix)

My nice finding with my previous company MyHosting .com

This error appears when the permissions on the user mail directories are incorrect, which usually shows up after a transfer. You can check the permissions under /var/qmail (even if the mail server is postfix), and his is what correct permissions look like:

# ll /var/qmail
total 8
drwxr-xr-x 7 root    root    4096 Jun 15 18:15 mailnames
drwxr-xr-x 3 popuser popuser 4096 Jun  2 02:48 popuser

# ll /var/qmail/mailnames/
total 16
drwxr-x--- 6 popuser popuser 4096 Jun  2 18:33 plesknew.prepgator.com
drwxr-x--- 2 popuser popuser 4096 Jun 15 18:09 test1.plesknew.prepgator.com
drwxr-x--- 2 popuser popuser 4096 Jun 15 18:09 test2.plesknew.prepgator.com

To fix it, you only need to fix the permissions:

chown root.root /var/qmail
chown root.root /var/qmail/mailnames
chown -R root.root /var/qmail/mailnames/.spamtrain
chown -R popuser.popuser /var/qmail/mailnames/*

Watch the mail log while you send mail to an account, as there may be other problems hiding behind this one.