Sieve is a dovecot plugin which allows a user to sort mail automatically into different mailboxes based on custom filters. The /etc/dovecot.conf file looks for sieve scripts in your home directory in a file called ".sieve". Basic documentation may be found here http://wiki.dovecot.org/LDA/Sieve.
There is a module for sieve called "vacation" which allows a user to auto reply to emails. This module will only execute the auto reply if the recipient of the email matches the user. Since we receive email on einstein (firstname.lastname@example.org), but it's addressed to email@example.com, the module will not autoreply by default. This problem is fixed by adding the ":addresses" clause in your sieve script to specify firstname.lastname@example.org as a recipient address you will send an autoreply for.
Additional documentation on sieve problems may be found here: http://www.dovecot.org/list/dovecot/2009-July/040949.html and here: http://geckoblue.livejournal.com/276001.html