If your sSMTP is configured to use gmail is giving you the following error:

send-mail: Cannot open smtp.gmail.com:587  

and in /var/log/maillog you see:

sSMTP: SSL not working: certificate verify failed (20)  
sSMTP: Cannot open smtp.gmail.com:587  

adding below line to /etc/ssmtp/ssmtp.conf should fix you right up:

TLS_CA_File=/etc/pki/tls/certs/ca-bundle.crt  

you should now find things succeeding in the logs:

sSMTP: Creating SSL connection to host  
sSMTP: SSL connection using RC4-SHA  
sSMTP: Sent mail for foo@example.com (221 2.0.0 closing connection - gsmtp) uid=0 outbytes=500  

Magic!