We recently ran into this error while trying to get a mail service working from within PHP. Our email service provider requires TLS security to validate our account before we are allowed to send mail through their system. This is a good practice as it keeps rogue email traffic from flooding the outbound network queue. However even after installing Mail and Net_SMTP from pear we still could not add mail to the outbound mail queue.
The solution was easy once we located the issue. It turns out the new PHP installation on a development box was compiled without OpenSSL libraries. The fix was to re-compile the source after configuring the build with openSSL enabled:
# ./configure <other-options-here> –with-openssl
We then restarted our Apache server to pick up the new PHP executable & the pear Mail libs worked perfectly with the SSL auth keys enabled.