I'm using Ubuntu 14.04, Dovecot, Postfix and Thunderbird. I have set up Postfix to use Maildir and I've set the following in the dovecot.conf for my mail location:

mail_location = maildir:/home/%u/Maildir

However, when I try to open my inbox in Thunderbird I receive the following error in the mail.log:

Error: open(/var/mail/mailuser1) failed: Permission denied (euid=3000(mailuser1) egid=3000(gruppe1)
missing +w perm: /var/mail, we're not in group 8(mail), dir owned by 0:8 mode=0775)

Why is it trying to open var/mail/mailuser1 when I specified in the config that the inbox is in /home/%u/Maildir? How do I fix this?