Symptoms
After migration from courier-IMAP service to Dovecot, mail is not getting delivered and maillog is being flooded with:
sieve: failed to stat user's sieve script: stat failed: Permission denied
Cause
A step with correct permissions' setting was skipped in the deployment guide.
Resolution
Set the permissions for /usr/libexec/dovecot/deliver
# chgrp qmail /usr/libexec/dovecot/deliver
# chmod g+s /usr/libexec/dovecot/deliver
# usermod -G qmail dovecot
As described in mail hosting deployment guide page 94.