A self-Signed SSL certificate is used along with PHP 5.6 to run the Horde webmail.
As a workaround, disable TLS encryption for sieve in the Horde configuration file:
Open the file /usr/share/psa-horde/ingo/config/backends.php
Find the section $backends['sieve'] = array(
Change the record:
'usetls' => true
to
'usetls' => false
Save the file with changes.
Create the additional Dovecot configuration file to specify "ssl=no" inside the "protocol sieve {}" section:
# cat /etc/dovecot/conf.d/99-horde-sieve-tlsfix.conf
protocol sieve { ssl = no
}
Restart Dovecot service:
# service dovecot restart