If you have a problem in sending email via smtp with pear.
and got this error message :
[message] => authentication failure [SMTP: Invalid response code received from server (code: 454, response: TLS currently unavailable)]
You need to change the user and group of exim.cert and exim.key to be mail:
chown mail:mail /etc/exim.cert
chown mail:mail /etc/exim.key