I added an email account to my email server recently. No problem.
My mail set-up is Arch Linux, Postfix and Dovecot. Thunderbird as a client using Imap.
The problem was, I was getting the following error:
Error: user newuser: Initialization failed: mail_location not set and autodetection failed: Mail storage autodetection failed with home=/home/newUser
The google machine didn't help.
It took a while, but the solution was to edit the file /etc/dovecot/conf.d/10-mail.conf
Add the line:
mail_location = mbox:~/mail:INBOX=/var/mail/%u
and restart dovecot. Problem solved.