After upgrading from Ubuntu 12.04 LTS to 12.04.1 LTS mail relay and delivery was no longer working.
The following errors were being reported in /var/log/mail.log:
Aug 26 16:51:47 mail postfix/smtpd: warning: SASL: Connect to private/auth failed: Connection refused
Aug 26 16:51:47 mail postfix/smtpd: fatal: no SASL authentication mechanisms
Aug 26 16:51:48 mail postfix/master: warning: process /usr/lib/postfix/smtpd pid 17398 exit status 1
Aug 26 16:51:48 mail postfix/master: warning: /usr/lib/postfix/smtpd: bad command startup -- throttling
After taking a closer look it seemed like 2 packages were missing:
sudo apt-get install dovecot-mysql dovecot-sieve
Something also broke the postfix/dovecot configuration:
tar -xzf ISPConfig-126.96.36.199.tar.gz
sudo php update.php
Now follow the installer/updater and choose to reconfigure services, once this is finished your mailserver should work again.